A secondary battery includes an electrode assembly, where the electrode assembly is a flat wound structure, the electrode assembly includes a first electrode sheet, a second electrode sheet, and a separator, the separator is disposed between the first electrode sheet and the second electrode sheet, and an innermost electrode sheet of the electrode assembly is the first electrode sheet; where a length of the electrode assembly is denoted as L, a width of the electrode assembly is denoted as W, a thickness of the electrode assembly is denoted as H, meeting 0.9≤L/W≤1.1 and H≤3 mm, the electrode assembly further includes an adhesive layer, an innermost turn of the first electrode sheet includes a first straight segment, a first bent segment, a second straight segment, and a second bent segment connected sequentially.