Abstract: In the field of nuclear arms control, non-proliferation is an significant area, particularly the proliferation of weapon usable nuclear materials, which is concerned by many countries. We used burnup calculating code MCORGS to simulate the relation between the burnup of spent fuel in PWR and Pu isotopic composition. We built the model of pin-cell with 20 average axial zones, calculated its axial burnup distribution, and got the axial burnup and Pu-239 isotopic mass concentration distribution in different axial positions in PWR. Based on burnup calculation we found that Pu-239 isotopic concentrations in different axial position varied greatly. Further more, we simulated VVER1000 and PWRs 1717 assembly. The simulation shows that the radial burnups in an assembly were also different. There are much different burnups in different positions of nuclear reactor, which result in many low burnup zones in spent fuel. This kind of spent fuel of LWR might brings serious nuclear proliferation to international community, and should be supervised more strictly.