To further study the chloride diffusion performance of cement paste and better consider the influence of cement particle shape characteristic， a hydration model of cement paste with accumulation of hydrated imperfect spherical particles was established. After fully verifying its reliability through third-party experiments， the influence of aspect ratio on hydration degree and pore structure feature was clarified. By applying the First-passage theory to the hydration model of cement paste， a Brownian motion algorithm was proposed for the diffusion characteristics of inner and outer hydration layer and unhydrated layer of the hydrated cement particle involved. By comparing the data from chloride diffusion test， the reliability of this algorithm was demonstrated. The influence of the aspect ratio of ellipsoidal cement particles on chloride diffusivity of cement paste was analyzed using the established Brownian motion algorithm. The results indicate that the established new method for analyzing the shape effect of cement particle on chloride diffusivity can provide references for the durability design of concrete.