Atomic Layer Deposition (ALD) is a gas-phase deposition technique based on surface self-limiting chemical reaction. Thin film is deposited on the substrate by streaming precursor and purge gas into the chamber alternatively. Compared with other film growth method such as chemical vapor deposition or physical vapor deposition, due to the self-limiting growth characteristic of ALD, it can realize the deposition of thin film on curved substrate with good conformality and uniformity without additional operations including rotating the substrate. ALD is expected to enable the growth of high-quality film on curved substrate. For example, glass X-ray monocapillary is an important X-ray optics and is a hollow tubular structure with the advantages of drawing into required shape easily and small roughness of inner surface. X-ray focusing beam can be obtained by the total reflection of incident X-rays on the inner surface of monocapillary. It, has been widely used in synchrotron radiation large-scale scientific system and X-ray analytical instruments. According to its working principle, the preparation of a uniform, smooth film with density higher than that of glass on the inner surface of monocapillary can further expand the application of the monocapillary in the field of high-energy X-rays.Although ALD has the potential to grow uniform film on the inner surface of monocapillary, the ALD process has to be optimized specially to achieve uniform film growth due to the large aspect ratio of monocapillary. It can be known that the key to preparing high-quality film on the inner surface of monocapillary is to ensure that the chemisorption of precursor is saturated on the entire inner surface and that the excess precursor or by-product is purged away by analyzing the process of growing thin film by ALD. Therefore, prolonging the precursor streaming time and purge time are commonly used. However, due to the fact that only a small portion of the precursor gas is used for film growth and a large portion of the precursor is discharged from the chamber outlet, prolonging the precursor streaming time will lead to a long preparation time and a waste of precursor.In this paper, we investigated a different way to obtain high-quality film on the inner surface of monocapillary without prolonging the precursor pass-through time. Firstly, three coating models were designed by adjusting the number of inlet and outlet in ALD chamber and the connection of monocapillary tube to the outlet. In Model 1 the monocapillary tube was placed in the center of the chamber without any other treatment. In Model 2 the monocapillary tube was placed on the outlet side of the chamber and connected to one of the outlets so that some of the gases had to pass through the monocapillary tube before being extracted from the connected outlets. In Model 3, the monocapillary tube was placed on the outlet side of the chamber and connected to two of the outlets and the rest of the outlets are all covered, that is, all of the extracted gases must pass through the monocapillary tube before leaving the chamber. Secondly, Fluent was used to numerically simulate the flow of gases through the monocapillary tube under three different models and the distribution, diffusion process of gases in the monocapillary were discussed respectively. Each ALD cycle of HfO₂ was carried out through a 9∶1 mixture of nitrogen and TDMAH pulse of 0.02 s, nitrogen purge of 30 s, a 9∶1 mixture of nitrogen and DI water pulse of 0.02 s and nitrogen purge of 30 s. Lastly, a model for the growth of high-quality HfO₂ film on the inner surface of monocapillary was obtained. Besides that, HfO₂ coated-monocapillary were prepared by ALD technique based on the three models and morphology of the films was characterized and analyzed.In brief, the process of growing HfO₂ thin film on the inner surface of monocapillary tube with large aspect ratio by ALD method was investigated using Fluent simulation. The flowing process of gases inside the monocapillary tube was simulated and analyzed under three different models. The steady-state simulation results show that compared with Model 1, there are enough precursors in Model 2 and 3 to realize the saturated chemisorption on the whole inner surface along the monocapillary and HfO₂ film with uniform thickness probably be obtained. The transient simulation results show that, compared with Model 3, excess precursors or by-product can be purged completely during the 30 s purging process in Model 2 and HfO₂ film with smooth surface probably be obtained. Therefore, the simulation results show that Model 2 is a better coating model suitable for growing high-quality film on the inner surface of monocapillary with large aspect ratio. It lays theoretical foundation for growing HfO₂ film by ALD method. The experimental results show that the HfO₂ film prepared under Model 1 has a smooth surface but uneven thickness, the HfO₂ film prepared under Model 2 has a smooth surface and a uniform thickness and the HfO₂ film prepared under Model 3 has a uniform thickness but a very rough surface. They are in agreement with the simulation results and analysis. It can be known that changing the number of inlets and outlets as well as the connection of monocapillary to the outlet is an effective and cost-saving way to improve the film quality on inner surface of monocapillary.