Monitoring of liquid phase diffusion process in real-time is very important in many fields such as chemistry, biomedicine, environmental science and so on. The traditional methods of liquid phase diffusion measurement mainly include nuclear magnetic resonance, optical holographic interferometry, diaphragm cell, etc. Fiber Bragg grating (FBG) has many advantages and especially can be used in harsh environment, so it has been widely used for sensing temperature, strain and refractive index. However, for ordinary FBG, the core mode is transmitted in the fiber core, so it can't be used directly for monitoring the liquid phase diffusion process. One method is coupling the light to the fiber cladding to excite the cladding modes, which are very sensitive to the refractive index. At present, there are three types of FBGs with strong cladding modes: tilted FBG (TFBG), off-axis FBG and high localized FBG (HLFBG). HLFBG is suitable for monitoring the liquid refractive index and liquid phase diffusion process and is usually inscribed by the femtosecond laser and point-by-point method, but high accuracy motorized linear translation stages are required.In this paper, a new monitoring method of liquid phase diffusion process based on HLFBG is proposed. The HLFBG is inscribed in hydrogen-loaded fiber by femtosecond laser and phase mask method. In order to reduce the peak power of femtosecond laser and avoid damage to the phase mask, the standard single-mode fiber is hydrogen loaded under 8 MPa and 120 °C for a week before inscription of FBG. A HLFBG with grating period of 1.071 μm is successfully inscribed by using femtosecond laser with a pulse energy of 400 μJ, a phase mask with period of 2.142 μm and a cylindrical lens with focal length of 40 mm. The width of the refractive index modulation region induced by femtosecond laser is about 4 μm, which is smaller than the fiber core diameter of 9 μm, so the refractive index modulation region of FBG can't completely cover the fiber core, resulting in high localization effect. The dip corresponding to the core mode in the transmission spectrum of the HLFBG is about -17.6 dB, besides, strong cladding modes with the dip of -17.2 dB and width of 145 nm are also excited, wich are extended from 1 403.42 nm to 1 548.28 nm. If the HLFBG is placed in air, most of the cladding modes are confined and transmitted in the cladding. If it's immersed in solution, when the refractive index of the solution outside the HLFBG is larger than some higher-order cladding modes, these higher-order cladding modes will change to leaky modes which are transmissted in the solution, so the leaky modes will loss quickly, besides, there is a cut-off mode between the the cladding modes and leaky modes. The central wavelength of the cut-off mode changes linearly with the change of the refractive index, and the refractive index sensitivity is measured to be 531.69 nm/RIU, while the core mode hardly changes with the refractive index, so it is suitable for temperature compensation in the liquid phase diffusion process. Due to its large refractive index measurement range, when the HLFBG is located at the interface of two liquids, the refractive index around HLFBG will change depending on the liquid phase diffusion process. By measuring the transmission spectra of HLFBG, the liquid phase diffusion process can be monitored in real time, and the relationship between the central wavelength shift of cut-off mode and liquid phase diffusion time is obtained. Futhermore, the concentration of the solution can be calculated according to the relationships between the central wavelength of cut-off mode, refractive index and concentration. From the experimental results it can be found that the liquid phase diffusion process of glycerin-water solution showed a double exponential function relationship. With the progress of diffusion process, the concentration of the solution around HLFBG begin to rise rapidly, and the diffusion process slows down gradually with the increase of diffusion time. The concentration of the glycerol solution reaches to about 41% after 565 min. The diffusion process will continue, but the diffusion rate will become slower and slower. So if the refractive index of a measured solution is lower than that of the fiber cladding, monitoring of the liquid phase diffusion process when it is mixed with other liquids can be realized by using this method, which will provide a new measurement means for the liquid mixing process in chemistry, biomedicine and other fields.