Burnup credit has an important impact on improving the efficiency of spent fuel storage. In the burnup credit, the burnup calculation model can affect the nuclide composition deviation, and the more accurate the nuclide composition, the lower the critical safety margin for spent fuel storage. To improve the accuracy of the burnup calculation, a multi-assembly burnup calculation model loaded with different fuel enrichment is proposed in this paper. Six samples of TMI-1 reactor NJ07OG assemblies were calculated, compared and analyzed by using different burnup calculation models. The results show that the average relative deviations of ²³⁵U, ²³⁸U and ²³⁹Pu obtained from the multi-assembly burnup model with different fuel enrichment are closer to zero and the relative deviations are more evenly distributed among the six samples than that of other models.