Abstract: In many books and papers about ERP published in China, the all kinds of core MRP logic models didn’t be described clearly and had defects in the logic in some way, which embarrassed the training of ERP students with higher quality. The influences of Past Due, Offset Time and Safety Time on the MRP logic model are analyzed in detail, which is in according with the international standard and practical demand. Finally an improved MRP logic model has been put forward which logic is more strict and expression is clearer.