A secondary battery and a battery unit is provided. The secondary battery includes: a housing including an accommodating hole with an opening; a top cover assembly connected to the housing in a sealed manner to cover and close the opening; an electrode assembly arranged in the accommodating hole, wherein the top cover assembly is spaced apart from the electrode assembly to form a first buffer gap for buffering the amount of expansion deformation of the electrode assembly in the axial direction. When the secondary battery expands, the first buffer gap can absorb the amount of expansion deformation to prevent the top cover assembly from being disconnected from the housing due to excessive stress, so as to reduce the possibility of failure of the secondary battery due to damage of the overall structure, thereby ensuring the structural integrity and the safety of the secondary battery.