The invention provides a large soft package battery module, an assembly method, a power battery and an electric vehicle. The large soft package battery module comprises a shell, buffer pads, a copperbar assembly, battery cells, a flexible circuit board and a heat-conducting fin, wherein the shell is provided with an opening in one side; the multiple layers of battery cells are stacked and then accommodated in the shell, the shell comprises a shell main body, an upper cover and a rear cover, the shell main body comprises a bottom plate and two side plates fixed at two opposite ends of the bottom plate, the upper cover covers the top of the multiple layers of battery cells and is fixedly connected with the two side plates, the rear cover covers one sides of the two side plates, and the heat-conducting fin is arranged between the rear cover and the multiple layers of battery cells; each buffer pad is arranged between two adjacent battery cells, the copper bar assembly comprises a plasticsupport and a copper bar fixed to the plastic support, and a guide groove is formed in the side, which deviates from the copper bar, of the plastic support; a tab of the battery cell penetrates through the plastic support and is connected with the copper bar, the flexible circuit board is buckled on the shell and is electrically connected with the copper bar, and a plastic cover plate fixedly connected with the shell covers the outer side of the copper bar. The large soft package battery module is high in energy density.