The present invention provides a constant volume bomb combustion gas mixing system and a control method. The system comprises gas cylinders of various types of gases, flow controllers, a gas mixer, a vacuum pump, a constant volume bomb, connecting pipes, a control part and a control circuit, wherein the connecting pipes are arranged between the gas cylinders, the flow controllers, the gas mixer, the vacuum pump and the constant volume bomb; the control circuit includes a vacuum pump driving circuit, a flow controller driving circuit, a signal acquisition circuit and a spark plug driving circuit; and the vacuum pump, the flow controllers, a pressure sensor which is arranged on the gas mixer, a micro pressure sensor, a pressure sensor which is arranged on the constant volume bomb, and a spark plug are all connected with the control part through the control circuit so as to be controlled, wherein the micro pressure sensor is arranged on the gas mixer, and the spark plug is arranged on the constant volume bomb. With the control method of the system adopted, precise molar volume ratio control on pre-combustion mixed gases can be realized, complete and controllable pre-combustion can be realized, a reliable environment can be provided for the search of the spray combustion characteristics of a diesel, experimental risk can be deceased, and high practicality can be realized.