The invention relates to a device for power transmission, transformation and storing for a motor vehicle, especially a high-efficiency energy-saving motor vehicle power device employing integration of mechanics, hydraulics and electronics to realize multi-power hybrid driving and stepless variable drive. The device is mostly composed of an engine, a low-power variable pump/motor, a high-power variable motor/pump, an electromotor/generator, a planet gear system, a locking device, a hydraulic energy storing device, a hydraulic controller, a converter/electronic control unit and battery unit, a hydraulic air conditioner compressor, etc. The invention has advantages of simple structure, easy manufacturing, convenient operation, low cost and small bulk. By employing special power control logics, the high-efficiency stepless transmission energy saving device provided by the invention can match the engine features rationally and optimizedly, force the engine work in the high-efficiency low-discharge area frequently, possess excellent dynamical features, realize parking stop and braking energy recovery, replace the present automobile transmission, improve the energy-saving feature of the present automobile and energy-saving driving feature of new automobile.