The invention provides a composite oil injector oil injection law measuring device and a measuring method. The measuring device mainly comprises an oil injector adapter, a volume fixing chamber, a volume fixing chamber oil drainage solenoid valve, a piston chamber, a piston, a piston chamber oil drainage solenoid valve, a pressure sensor, a temperature sensor, a displacement sensor and the like. The measuring device is further provided with an electronic control system, and the electronic control system comprises a controller, a data collecting card and a computer. According to the measuring method, an oil injection law curve is obtained through a volume method, accurate circulating oil injection mass is obtained through a displacement method, and therefore an oil injection law is corrected, and the oil injection law of an oil injector is obtained more accurately. Oil injection law testing can be carried out, the advantages of the volume method and the displacement method are combined, the oil injection law can be measured more accurately, and the composite oil injector oil injection law measuring device and the measuring method can be applied to the research and development work of engines.