Thermal diffusion coefficient is an important parameter of optical components in high-energy and high-power laser systems, and it is related to the laser damage resistance of components. However, the measurement error of the existing thermal diffusion coefficient measurement methods is large under the condition of multi-dimensional thermal conduction. As thermal diffusion length is the basis of thermal diffusion coefficient measurement, our study used the finite element method to simulate the two-dimensional heat conduction under continuous heating of heat source, and summarized the relationship between thermal diffusion length, thermal diffusion coefficient and heating time. On this basis, it proposed a model and method for measuring two-dimensional thermal diffusion length under continuous heating of heat source. Firstly, finite element analysis was used to establish a model to simulate the relationship between thermal diffusion length and thermal diffusion coefficient in one-dimensional heat conduction, and the two models were compared with numerical analytical expressions. The feasibility of using continuous heat source and thermal diffusion length to solve the thermal diffusion coefficient was verified. The effects of heat loss, sample thickness and heat source loading time on the results were discussed. Finally, the practical measurement scheme and measures to improve the measurement accuracy were put forward. This study provides a way to measure the thermal diffusion length of materials or components conveniently and accurately, and is of great significance for fabrication of high power and high energy laser system components.