Abstract: The core of HTR-PM which is fixed and connected with keys and dowels system, exhibits nonlinear characters under seismic loads. A seismic test of 1∶3 scaled side-reflector was conducted by Institute of Nuclear and New Energy Technology of Tsinghua University aiming to investigate the HTR-PM core nonlinear response. The model was made of graphite bricks, carbon bricks, graphite dowels and keys. Random loads, OBE loads, SSE loads were applied under conditions including lower-barrel without ceramic pebbles, lower-barrel with ceramic pebbles and whole model with ceramic pebbles. Thus, modes, acceleration dynamic responses and displacement dynamic responses are obtained. According with safety standards, the integrity of the HTR-PM core internals under seismic conditions is confirmed.