Studies of the driving force of the self-propulsion Janus particles are very important in the fields of micro-power and nano-motor. In this paper, we choose the micron Pt-SiO2-type Janus particle as a research object, which is propelled by self-generated concentration gradient in the dilute solution of H2O2, focusing on the self-propulsion of the single particle. According to the force analysis of the Janus particle, the surface force can be decomposed into the viscous resistance of the fluid, the Brownian force derived from the molecular thermal fluctuation, and the diffusiophoresis caused by the diffusion of the solute component. The main aim of this paper is to find the way to accurately simulate the diffusiophoresis generated by the huge concentration gradient on a microscale. The lattice Boltzmann method (LBM) is a modern mesoscopic method based on the microscopic particle characteristics of the fluid, which makes it more intuitive to deal with the interaction between the fluid and solid. It is more advantageous than the traditional numerical method in the description of this micro-interface dynamic problem, i.e., the self-propulsion of Janus particle. On a certain time scale, when the Janus particle shows the directional motion, the influence of the Brownian force can be ignored. Thus, the analytical process can be simplified. Based on the momentum theorem, the method of calculating the diffusiophoresis produced by concentration diffusion is proposed. We introduce the momentum exchange in the half-way bounce-back scheme of LBM into the model of the multicomponent diffusion and reaction. Through counting the surface force we can obtain the diffusiophoresis acting on the Janus particle. Moreover, this diffusiophoresis model is modified by comparing the experimental fluid resistance with simulated one. This comparision verifies the validity of the diffusiophoresis model. Then, the analysis of the variation of diffusiophoresis proves that the value of diffusiophoresis is independent of the fluid velocity. Through the further application of this model, the different shapes of Janus particles with the same volume are compared in simulations. The results show that the self-diffusiophoresis is mainly determined by the axial projection area. In addition, the reaction area of the particle also affects the value of the diffusiophoresis.