The present disclosure relates to a secondary battery and a battery module. The secondary battery comprises a case, comprising a base plate and a side plate connected with the base plate, wherein the base plate and the side plate form a receiving hole and an opening in communication with the receiving hole, the opening is arranged opposite to the base plate in an axial direction of the receiving hole, and the base plate has a thickness larger than that of the side plate; a cap assembly, sealingly connected with the side plate to close the opening; and an electrode assembly, disposed in the receiving hole and comprising two or more electrode units, which are stacked in the axial direction, and each electrode unit is arranged with a wide side opposite to the base plate and a narrow side toward the side plate.