Abstract: Based on the finite difference method, the mechanical model of multi-layer cylindrical body was established for accident tolerant fuel (ATF). The new model was implemented in FRAPCON 4.0 code to replace the original Fracas model. By the simulation for the same research objective, the characteristics of the new and old models were analyzed and the rationality and advancement of modification were validated. The comparison indicates that most of parameters agree well, demonstrating that the non-rigid fuel assumption is reasonable to some extent for the traditional fuel. However, the old model is over conservative in contact pressure prediction, and the upgraded code provides more reasonable result. Finally, the potential application of the multi-layer mechanical model and the upgraded FRAPCON 4.0 in the fuel behavior analysis was predicted.