Abstract: Thermal gas mass flowmeter has many advantages, such as large range ratio, high precision, and reliability. It can be widely used in the measurement of industrial natural gas, household gas, and oxygen in medical treatment. It has a broad application prospect in industrial and civil fields. At present, the maximum flow rate of low-pressure natural gas practical flow measurement standard device is usually only 40 m³·h⁻¹, which is unable to test the low-pressure high-flow gas mass flowmeter, bringing a lot of inconvenience to the production. In this paper, the thermal gas mass flowmeter is modified by using the split flow component, and the flow field characteristics of the split flow component and the whole meter are compared and verified by numerical simulation. The practical flow test equipment is used to carry out the practical flow test of the split flow component, and the flow rate performance of the flowmeter is obtained by using the flow proportional coefficient obtained by calculation, thus solving the practical flow test problem of the low-pressure high-flow gas mass flowmeter.