A power supply unit for an electric vehicle, comprising: a tray; at least one battery module, each battery module including: a housing having a bottom plate mounted onto the tray and first to fourth side plates (10, 17, 3, 15) disposed on the bottom plate, the first and third side plates (10, 3) being opposed to each other in a first direction, the second and fourth side plates (17, 15) being opposed to each other in a second direction; and a battery pack disposed in the housing and having a plurality of cells arranged along the first 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, characterized in that the at least one battery module is fixed on the tray via a strip and the housing further comprises a plurality of support bars (9, 6, 1, 13, 8, 12) configured to fasten the first to fourth side plates to the bottom plate, wherein the support bars comprise first to sixth support bars (9, 6, 1, 13, 8, 12), wherein the first and fourth support bars (9, 13) are disposed on the two ends of the first side plate in the second direction respectively, the second and third support bars (6, 1) are disposed on the two ends of the third side plate in the second direction respectively, the fifth support bar is connected between the first and second support bars (9,6), and the sixth support bar is connected between the third and fourth support bars (1, 13), wherein the strip is in the form of a belt, one end of which is fixed onto the tray, and the other end thereof passes over the first side plate, a top surface of the battery module and the third side plate in turn, and finally is fixed to the tray, wherein projections (7, 18) each having an inclined surface are disposed on outside surfaces of the first and third side plates (10, 3) respectively, the other end of the strip is passed over the projection having the inclined surface of the first side plate, the top surface of the battery module and the projection having the inclined surface of the third side plate in turn, and finally fixed to the tray. a tray; at least one battery module, each battery module including: a housing having a bottom plate mounted onto the tray and first to fourth side plates (10, 17, 3, 15) disposed on the bottom plate, the first and third side plates (10, 3) being opposed to each other in a first direction, the second and fourth side plates (17, 15) being opposed to each other in a second direction; and a battery pack disposed in the housing and having a plurality of cells arranged along the first 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, characterized in that the at least one battery module is fixed on the tray via a strip and the housing further comprises a plurality of support bars (9, 6, 1, 13, 8, 12) configured to fasten the first to fourth side plates to the bottom plate, a housing having a bottom plate mounted onto the tray and first to fourth side plates (10, 17, 3, 15) disposed on the bottom plate, the first and third side plates (10, 3) being opposed to each other in a first direction, the second and fourth side plates (17, 15) being opposed to each other in a second direction; and a battery pack disposed in the housing and having a plurality of cells arranged along the first 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, characterized in that the at least one battery module is fixed on the tray via a strip and the housing further comprises a plurality of support bars (9, 6, 1, 13, 8, 12) configured to fasten the first to fourth side plates to the bottom plate, wherein the support bars comprise first to sixth support bars (9, 6, 1, 13, 8, 12), wherein the first and fourth support bars (9, 13) are disposed on the two ends of the first side plate in the second direction respectively, the second and third support bars (6, 1) are disposed on the two ends of the third side plate in the second direction respectively, the fifth support bar is connected between the first and second support bars (9,6), and the sixth support bar is connected between the third and fourth support bars (1, 13), wherein the strip is in the form of a belt, one end of which is fixed onto the tray, and the other end thereof passes over the first side plate, a top surface of the battery module and the third side plate in turn, and finally is fixed to the tray, wherein projections (7, 18) each having an inclined surface are disposed on outside surfaces of the first and third side plates (10, 3) respectively, the other end of the strip is passed over the projection having the inclined surface of the first side plate, the top surface of the battery module and the projection having the inclined surface of the third side plate in turn, and finally fixed to the tray.