Abstract: This paper is theoretical studies about the method of the pressure stimula ted by gas injecting(PSGI),which was nominated for the most suitable candidate applicable to the chinese telecommunication satellite to measure liquid propellant remaining.Two main thermodynamical models,the isothermal model and the adiabatic model,were established and the dominant equations corresponding to the models were derived for practical measurement usage.Lastly,some problems associated with actual operations were discussed in details.