The physical processes of remainder-propellant exhausting from the tank of the third stage rocket after the separation of SatelliteRocket for a CZ-4B launch vehicle are analyzed by the vacuum technigue.The probable contamination for FY-1 satellite and SJ-5 carrier satellite from the exhausting propellant are computed.The paper discribes the basic assumptions, physical modeI and numerical method.The maximum contamination accumulated on the satellite and auxiliary satellite during the propellant exhausting process has been reasonably analyzed and obtained.