Abstract: To estimate directly the residual mass of the satellite gas propellant under a high pressure, different methods of the gas density calculation are used. It is found that with the calculation method based on the ideal gas state equation for the gaseous propellant gauging system, some inherent error will be involved under a high pressure. The results of the Redliche-Kwong (RK) equation are very close to those obtained by the NIST-based method. An explicit method using the gas density-temperature-pressure fitting expression is obtained based on the NIST database, and a tank volume linear-correction model is proposed. Experimental results show that the axial deformation length increases linearly with the tank pressure, which shows the correctness of the model. The NIST-based explicit method can best fit the propellant filling data, and the volume correction model can reduce the error efficiently. For a satellite cold gas system, the uncertainty of the propellant filling is 180 g, equal to 0.62% in relative uncertainty.