Abstract: In order to optimize the nuclear material accounting of spent fuel reprocessing facilities and find the main factors of the material unaccounted for (MUF), a numerical simulation model based on nuclear material accounting perspective is built, which includes the front-end process, PUREX process, uranium and plutonium back-end and MUF calculations. The material flow rate in the model can be changed easily by changing different process parameters to simulate different production scale. The simulated material measurement error comprising of random and system errors are simulated by normal distributed random variables and constant numbers respectively, which have the similar randomness and statistical features with the actual measurement uncertainty. When the model measurements are set to different random and system errors, the corresponding square error distribution and improvement direction can be obtained, by which a better measurements precision combination satisfying the uncertainty requirement can be found. The final results show that the system error of the 1AF measurement is the main source of the MUF.