Abstract: Based on the Monte Carlo method for burnup calculations, the errors increase as a function of burnup. The errors of Monte Carlo burnup calculations were studied in this paper, the modification measures were used which could improve the accuracy of burnup calculation. The results show that, the effect of statistical error propagation would be reduced by unbiased minimum variance (UMV) modification, the exactitude of Monte Carlo transport calculation would be ensured by density modification, and the accuracy would be improved further by local optimization of burnup cross-section library, which create conditions for engineering application.