Abstract: Fast reactor system analysis code FASYS is developed by China Institute of Atomic Energy, which has been used for commissioning test analysis of China Experimental Fast Reactor and is currently being used for accident analysis of China Demonstration Fast Reactor. The FASYS code includes core analysis module, primary and secondary loop modules, and decay heat removal system module, etc. The core analysis module includes point reactor model, decay heat model, reactivity feedback model, and thermal-hydraulic model of core channel, etc. The point reactor model of FASYS code was validated by comparing with analytical solution result, DINROS code result and SAS4A/SASSYS-1 code result. And decay heat model, reactivity feedback model and core channel thermal-hydraulic model of FASYS code were validated by comparing with SAS4A/SASSYS-1 code result. The validation results of each model are in good agreement. The calculation deviation of each model for the core analysis module of FASYS code was evaluated. And the proposal for China Demonstration Fast Reactor accident analysis deviation evaluation was given.