To balance IRE for deep penetration, it is necessary for nuclear power plants (NPPs) operates with enough flexibility. To realize the flexible operation, the automatic generation control (AGC) should be developed. In this paper, the adaptive AGC is proposed for SMR-based multimodular NPPs, which has a simple proportional-integral (PI) structure. The corresponding control system of multimodular NPPs with AGC function is also designed, which is constituted by the units of NSSS control, multimodular coordination, turbine speed governor (TSG) and AGC. The adaptive AGC method as well as the plant control system design with AGC function are then applied to two modular high temperature gas-cooled reactor plant HTR-PM. Numerical simulation results show that the newly-built multimodular control system can well stabilize the grid frequency by properly performing the power-level maneuver and maintenance of NSSS modules.