The paper used tracer gas concentration to analyze the entrainment movement of ambient air by the non-isothermal cold jet coming from the circular nozzle. The model experiment of nozzle jet has been conducted in the thermostatic and humidistatic laboratory.It compared the result of the entrainment volumes derived from the traditional method and the tracer gas method, and analyzed the patterns of entrainment volume and the entrainment coefficient along the jet curve. The results show that the method of measuring the jet entrainment according to on-way changes of tracer gas concentration is feasible. The Reynolds number is the dominant physical factors in the movement of jet where the inertial force plays a dominant role. With the increase of Re number, the volume of entrainment increases gradually, when Re＞15000, the flow increase coefficient almost keep the same.