The present disclosure provides a secondary battery, a battery module and a vehicle. The secondary battery includes an electrode assembly, a case and a cap assembly. The case has an accommodating cavity, and the electrode assembly is accommodated in the accommodating cavity. The electrode assembly includes electrode units, and the electrode units are stacked in an axial direction of the accommodating cavity. The cap assembly includes a cap plate and an insulating member provided at an inner side of the cap plate, the cap plate is connected with the case, and the insulating member is positioned at a side of the electrode assembly in the axial direction. The insulating member is provided with a first surface at a side close to the electrode assembly, and the first surface is a flat surface.