Abstract: In Monte Carlo calculation, the geometrical precision of model is equilibrium between calculation efficiency and accuracy of result. The precise modeling of reactor core largely decreases calculation efficiency, while homogenization by experience brings large deviation to the result. In this paper, the influence of geometry simplification of core structures of China Experimental Fast Reactor on keff was assessed based on eigenvalue-difference method, and an optimized core model was established. The results indicate that the optimized core model largely improves calculation efficiency with the result remaining almost the same. The detailed geometrical treatment and calculation results provide reference for Monte Carlo model in reactor core design and experiment analysis, and present academic worth to Monte Carlo code development and theoretical research.