The invention relates to an assembly method of a square multi-tab winding lithium ion battery. The method comprises the following steps: (1) die cutting of a pole piece, namely, controlling the die cutting size, and arranging tabs at corners at two sides of a winding core; (2) pole piece winding, namely, winding the pole piece and arranging the pole piece at the corners at two sides of the windingcore; (3) sizing of the winding core, namely, sizing the winding core in a mode of thermally pressing and then cold pressing; (4) kneading of the winding core, namely, kneading the corners at two sides of the winding core; (5) laser welding, namely, welding the kneaded winding core and one end of a current collecting body through laser, and welding the other end of the current collecting body anda cover plate through laser; (6) coating with an insulation film; and (7) mounting into a housing. With the adoption of the method, the occupied space of the current collecting body and a welding station is small; the occupied spaces of the tabs are extremely small, so that the effective volume ratio of the winding core is increased, and the battery capacity and the energy density are improved; the shape of the current collecting body is free from special requirement by being compared with a traditional technology; the welding operation is simple; and moreover, the product production efficiency is high.