Throttling Venturi for cryogenic propellants is the key component of the mass flow control system. The step motor was used to control the displacement of the tapered rod. And water is used in cold test. Through changing operating conditions, throat area and back pressure, the change rule of discharge coefficient and pressure recovery factor were found. Then liquid oxygen was used to modify the results of cold test. The mass flow control system was used in firing test of tripropellant model engine, which shows the system can control the mass flow of liquid oxygen accurately and change the operating conditions successfully.