Abstract: The formulas of leak sensibility based on the leak rates of the atmospheric partial pressures are given.The conditions to obtain the high leak sensibility are analyzed and the synthe-tical factors to select the detectable gas is proposed.It is shown from the calculation that argon is a very sensitive gas for detecting the leak of the sputter ion pumped system and it is sensitive enough to assure the leak rate of the detected leak much less than that of the gas release of the system. The detection result under 10^-8 Pa is also given，which is in good agreement with the theory.