A gearbox casing, a bottom part or a side part of the gearbox casing is provided with a number of first coolant tanks, the gearbox casing above a bottom part of the first coolant tanks is provided therein with a lubricating liquid, and the first coolant tanks are used to cool the lubricating liquid; one of the first coolant tanks is provided therein with a number of parallel partition walls, by which the one of the first coolant tanks is separated into at least two communicated sub-tanks; a plurality of first fixed guide ribs interlaced with each other are projected from the partition walls or an inner wall of the one of the first coolant tanks at two sides of each of the sub-tanks respectively, thereby forming a continuous S-shaped or maze shaped channel for the coolant to flow through; the one of the first coolant tanks is provided thereon with a first liquid inlet and a first liquid outlet, which are provided at one end or two ends of the one of the first coolant tanks; first suspended guide ribs are isolatedly provided between two adjacent first fixed guide ribs, between the first fixed guide rib and the first liquid inlet, and/or between the first fixed guide rib and the first liquid outlet, the bottom part or a side end surface of the gearbox casing is further provided with a second coolant tank to cool a side interior of the gearbox casing; a second cover plate is provided on the second coolant tank, two ends of the second coolant tank are respectively provided with a second liquid inlet and a second liquid outlet, and the first liquid outlet is connected with the second liquid inlet, characterized in that: the second coolant tank is provided therein with a plurality of second fixed guide ribs, and two adjacent second fixed guide ribs are respectively connected with two opposite inner walls of the second coolant tank, so that the coolant flows between the plurality of second fixed guide ribs in a continuous S or maze shape; second suspended guide ribs are isolatedly provided between two adjacent second fixed guide ribs, between the second fixed guide rib and the second liquid inlet, and/or between the second fixed guide rib and the second liquid outlet. one of the first coolant tanks is provided therein with a number of parallel partition walls, by which the one of the first coolant tanks is separated into at least two communicated sub-tanks; a plurality of first fixed guide ribs interlaced with each other are projected from the partition walls or an inner wall of the one of the first coolant tanks at two sides of each of the sub-tanks respectively, thereby forming a continuous S-shaped or maze shaped channel for the coolant to flow through; the one of the first coolant tanks is provided thereon with a first liquid inlet and a first liquid outlet, which are provided at one end or two ends of the one of the first coolant tanks; first suspended guide ribs are isolatedly provided between two adjacent first fixed guide ribs, between the first fixed guide rib and the first liquid inlet, and/or between the first fixed guide rib and the first liquid outlet, the bottom part or a side end surface of the gearbox casing is further provided with a second coolant tank to cool a side interior of the gearbox casing; a second cover plate is provided on the second coolant tank, two ends of the second coolant tank are respectively provided with a second liquid inlet and a second liquid outlet, and the first liquid outlet is connected with the second liquid inlet, characterized in that: the second coolant tank is provided therein with a plurality of second fixed guide ribs, and two adjacent second fixed guide ribs are respectively connected with two opposite inner walls of the second coolant tank, so that the coolant flows between the plurality of second fixed guide ribs in a continuous S or maze shape; second suspended guide ribs are isolatedly provided between two adjacent second fixed guide ribs, between the second fixed guide rib and the second liquid inlet, and/or between the second fixed guide rib and the second liquid outlet.