The invention belongs to the technical field of automotive suspensions and provides an oil-gas suspension with a volume-variable additional gas chamber and a control method of the oil-gas suspension. The oil-gas suspension comprises a detection mechanism, an execution mechanism, the additional gas chamber, an additional hydraulic cylinder, an oil-gas suspension body, an oil tank and an electronic control unit. By detecting the vehicle speed, the vehicle body vertical acceleration, the vehicle transverse acceleration and liquid pressure signals of a lower cavity of the additional gas chamber and a high-pressure oil storage tank, the electronic control unit achieves adjustment of the pressure of hydraulic oil in the additional gas chamber by controlling the execution mechanism; and high-pressure hydraulic oil is generated by the additional hydraulic cylinder through energy generated during vibration of a vehicle, so that pressure of nitrogen in the additional gas chamber is adjusted, and accordingly the rigidity of the oil-gas suspension body is adjusted. The additional hydraulic cylinder can achieve the functions of a shock absorber, and meanwhile energy waste is reduced. According to the oil-gas suspension with the volume-variable additional gas chamber and the control method of the oil-gas suspension, the rigidity adjustment range is wide; and meanwhile, due to the fact that throttling holes have a damping function, the smoothness of the vehicle is better, and the riding comfort is improved advantageously.