Abstract: Flow velocity and flowrate of gas will be certainly affected under the condition of choked flow. Based on gas dynamics, steady state simulation model is established by means of BWRS equation of state. Calculation is made on maximum flowrate of choked flow under the condition of constant section pipe. Taking the 10 m-long gas loading and unloading high pressure sebific duct of CNG tanker as an example, the maximum flowrate flowing through at different conditions is solved, and variation laws of parameters such as gas flow rate, pressure, Mach number and temperature along pipeline are discussed.