A battery module assembly comprising unit modules, each of which is configured such that unit cells are loaded in a cartridge in a state in which the unit cells are electrically connected to each other via a bus bar, the battery module assembly comprising: two or more sub modules arranged in a lateral direction, each of the sub modules comprising two or more unit modules stacked from a ground in a vertical direction; a base plate on which the sub modules are loaded; side cover plates mounted at sides of the sub modules; an upper cover plate loaded on tops of the sub modules, the upper cover plate being fastened and coupled to the sub modules and the side cover plates; a front cover plate mounted at a front of the battery module assembly at which external input and output terminals (161,162) of the sub modules are located, the front cover plate being fastened and coupled to the sub modules, the base plate, and the side cover plates; a main bus bar for fixing front ends of the sub modules to each other and connecting the external input and output terminals (161, 162) of the sub modules; and a bracket for fixing rear ends of the sub modules to each other, wherein the unit cells in each of the unit modules are connected in parallel to each other, and the unit cells are mounted on the cartridge such that electrode terminals of the unit cells (110a, 110b) face each other, and the electrode terminals of the unit cells (110a, 110b) are connected in parallel to each other by bus bars mounted at the cartridge, the unit modules are stacked such that a coolant flow channel is formed at a stacked interface between the unit modules, each of the side cover plates is provided with one or more coolant introduction holes for allowing a coolant to be introduced therethrough, and the bracket is provided with a coolant discharge hole communicating with the coolant flow channel. two or more sub modules arranged in a lateral direction, each of the sub modules comprising two or more unit modules stacked from a ground in a vertical direction; a base plate on which the sub modules are loaded; side cover plates mounted at sides of the sub modules; an upper cover plate loaded on tops of the sub modules, the upper cover plate being fastened and coupled to the sub modules and the side cover plates; a front cover plate mounted at a front of the battery module assembly at which external input and output terminals (161,162) of the sub modules are located, the front cover plate being fastened and coupled to the sub modules, the base plate, and the side cover plates; a main bus bar for fixing front ends of the sub modules to each other and connecting the external input and output terminals (161, 162) of the sub modules; and a bracket for fixing rear ends of the sub modules to each other, wherein the unit cells in each of the unit modules are connected in parallel to each other, and the unit cells are mounted on the cartridge such that electrode terminals of the unit cells (110a, 110b) face each other, and the electrode terminals of the unit cells (110a, 110b) are connected in parallel to each other by bus bars mounted at the cartridge, the unit modules are stacked such that a coolant flow channel is formed at a stacked interface between the unit modules, each of the side cover plates is provided with one or more coolant introduction holes for allowing a coolant to be introduced therethrough, and the bracket is provided with a coolant discharge hole communicating with the coolant flow channel.