Abstract: The influence of the direct and bypass gas-sampling modes on leak detection in-situ,with mass spectroscopy in atmospheric environment,of the spacecraft facilities,including the propelling/thermal/environment control units and fluid media loops,was experimentally investigated.First,the direct and bypass sampling modes of tracer gases were mathematically modeled and theoretically analyzed.Second,the dependence of He,Kr,Xe and CF₄ background pressures on the leak-detector chamber pressure was repeatedly measured to improve the stability and sensitivity.Third,the bypass sampling mode was found to outperform the direct sampling mode.And finally,the calibration coefficients with relation to the background signal were derived by repeatedly evaluating in bypass sampling mode the standard leaks of He,Kr and Xe tracers.The sensitivity of the newly-developed in-situ leak detection technique was estimated to be better than 1.0×10^-8 Pa·m³/s,meeting the stringent requirements for leak detection of spacecraft.