Abstract: Casing once-through steam generator (COTSG) was used in the innovative compact nuclear power. The dynamic characteristics of the COTSG were analyzed by means of the nuclear power system analysis code SCDAP/RELAP5/MOD3.4. The primary coolant average temperature constant method and the secondary steam pressure constant method operation mode were adopted with load following controlling mode. The dynamic response processes of rapidly decreasing secondary load were discussed in detail. The steam pressure constant can be satisfied by optimizing the PID controller parameters through exactly controlling feed water mass flow rate. The process of decreasing or increasing load is very quickly and the steam pressure is stable. The dynamic response time is very short during the dynamic processes.