Abstract: The effective and convenient method to measure mass flow rate is absent now when absolute gas pressure less than 40 kPa and gas temperature from negative 70 to positive 80℃. For this case, a flow rate measurement method based on dynamic equilibrium principle was proposed and a novel flow metering device was designed. In low pressure and wide temperature, the flow rate through a control valve is determined by four factors: upstream pressure of valve, downstream pressure of valve, temperature and valve opening. With the data samples obtained from the metering system, the expression of the relationship between the flow rate and the four factors was established based on BP neural network method. The experimental results indicated the relative error of flow rate calculation was less than 1.8% and the proposed flow rate measurement method was effective.