The invention provides an energy-saving and emission-reducing device for a vehicle. According to the specific scheme, vehicle gravity comes from gravitational attraction and becomes vertically downwards gravity; in the rolling process of a wheel, gravity on the rear portion of the wheel bears mechanical power of the vehicle, and the lifted gravity has the tendency of negative work movement; when the lifted gravity strides across the center vertical line of the wheel, the gravity of the wheel and the vehicle gravity which is continuously transmitted are integrated on the front portion of the wheel to form resultant force, the resultant force makes positive work movement with the gravity decreased, and the gravity is converted into kinetic energy during the movement; a large gear ring combined with a hub and arranged on the inner side of the wheel decomposes and obtains component force during rolling of the wheel, that is, part of the kinetic energy is transmitted by a transmission device and used for operation and power generation of a power generator of the vehicle, electricity used by the vehicle in the prior art is replaced, power for starting a diesel engine of the power drives the power generator to operate and generate power for the vehicle to use, and energy conservation and emission reduction of the vehicle are achieved.