A method for measuring respective flowrates of gas phase and liquid phase in a multiphase fluid using a critical flow nozzle flowmeter. The critical flow nozzle flowmeter includes a throttling nozzle having an inlet, an outlet and a throat, and the throat has a smallest flow area for flowing fluid; a gamma ray detector, including a gamma ray emitter and a gamma ray receiver, arranged in a way allowing the gamma ray emitted by the gamma ray emitter to pass through a cross-section at the inlet of the throttling nozzle in a diametrical direction to reach the gamma ray receiver; pressure sensors respectively configured for measuring the pressure P1 at the inlet of the throttling zone and the pressure P2 at the outlet of the throttling nozzle; and a temperature sensor configured for measuring the temperature T1 at the inlet of the throttling nozzle.