The net power of an automobile engine is one of the important indicators for measuring engine performance, which reflects the power output capacity of the engine under actual working conditions. In this paper, a natural gas engine is selected. Based on the test procedure and measurement method, a mathematical model of net power is established. The standard uncertainties of parameters such as measurement repeatability, measured engine torque, measured engine speed, atmospheric pressure, intake humidity, and intake temperature in the model are analyzed. Finally, the evaluation result of measurement uncertainty of net power is obtained.