To further study the chloride diffusion performance in cement paste and better take into account the influence of shape characteristic of cement particle, the vector model of cement paste with accumulation of hydrated imperfectly spherical particles was firstly established in this paper. After the validity of this hydration model was verified by the third-party experiment, the influence of aspect ratio on hydration degree and pore structure feature was quantified. Secondly, by applying the first-passage theory to the reconstructed microstructure model of cement paste, the Brownian motion algorithm was proposed with the diffusion characteristics of inner and outer hydration layer and unhydrated layer of the hydrated cement particle involved. By comparison of the data from chloride diffusion test, the reliability of this algorithm was demonstrated. At last, the influence of the aspect ratio of ellipsoidal cement particles on chloride diffusivity of cement paste was drawn. This study gave a new method for analyzing the shape effect of cement particle on chloride diffusivity, which could provide some reference for durability design of concrete.