A power supply unit for an electric vehicle includes a tray and at least one battery module fixed on the tray via a strip. Each battery module includes a housing having a bottom plate mounted onto the tray, first to fourth side plates disposed on the bottom plate, a battery pack disposed in the housing and having a plurality of cells arranged along a thickness direction, and first flexible members disposed between the first side plate and the battery pack, and between the third side plate and battery pack respectively, for fastening the battery pack.