A cap plate configured to be comprised by a secondary battery, wherein the cap plate is rectangular and includes a main portion and a first protruding portion; the main portion includes a first exterior surface, a second exterior surface and a third exterior surface which are formed along a periphery of the main portion, the third exterior surface is a curved surface and connects the first exterior surface and the second exterior surface; the first exterior surface is planar and is provided as two in number, the second exterior surface is planar and is provided as two in number, the first exterior surfaces and the second exterior surfaces are alternately provided along the periphery of the main portion, the third exterior surfaces is provided as four in number, the third exterior surfaces each are provided between each first exterior surface and each second exterior surface adjacent to the first exterior surface, the third exterior surfaces each is positioned at a corner of the cap plate; the first protruding portion is provided as four in number and positioned below the main portion, and each first protruding portion includes a fourth exterior surface extending downwardly from a bottom end of one corresponding third exterior surface, each fourth exterior surface is inclined inwardly relative to the one corresponding third exterior surface; the secondary battery further comprises a case; the case includes an opening in a top thereof, a shape of the opening is fitted with a shape of the periphery of the main portion; the cap plate is provided to the opening of the case, and the main portion of the cap plate is fixed to an inner wall of the case; the first protruding portion of the cap plate is received in the case, and a gap is provided between the fourth exterior surface and the inner wall of the case, a dimension of the gap increases gradually along a downward direction; the opening of the case is expanded when the first protruding portion of the cap plate is received in the case. the cap plate is rectangular and includes a main portion and a first protruding portion; the main portion includes a first exterior surface, a second exterior surface and a third exterior surface which are formed along a periphery of the main portion, the third exterior surface is a curved surface and connects the first exterior surface and the second exterior surface; the first exterior surface is planar and is provided as two in number, the second exterior surface is planar and is provided as two in number, the first exterior surfaces and the second exterior surfaces are alternately provided along the periphery of the main portion, the third exterior surfaces is provided as four in number, the third exterior surfaces each are provided between each first exterior surface and each second exterior surface adjacent to the first exterior surface, the third exterior surfaces each is positioned at a corner of the cap plate; the first protruding portion is provided as four in number and positioned below the main portion, and each first protruding portion includes a fourth exterior surface extending downwardly from a bottom end of one corresponding third exterior surface, each fourth exterior surface is inclined inwardly relative to the one corresponding third exterior surface; the secondary battery further comprises a case; the case includes an opening in a top thereof, a shape of the opening is fitted with a shape of the periphery of the main portion; the cap plate is provided to the opening of the case, and the main portion of the cap plate is fixed to an inner wall of the case; the first protruding portion of the cap plate is received in the case, and a gap is provided between the fourth exterior surface and the inner wall of the case, a dimension of the gap increases gradually along a downward direction; the opening of the case is expanded when the first protruding portion of the cap plate is received in the case.