Abstract: We addressed the uncertainty-evaluation method,in leak-rate testing for spacecraft propulsion unit by accumulation leak detection technique with helium mass spectrometer in ambient environment.First,the conventional Type-A/B uncertainties were mathematically formulated with Linear-Equation in One-Variable model.Next,the influence of Type-A/B uncertainties on the measured leak-rate uncertainty,obtained in helium accumulation and in leak-rate calibration,was analyzed in Least-Square algorithm.The results show that when it comes to leak-rate uncertainty in helium accumulation,Type-B uncertainty has a much greater impact than Type-A uncertainty does.And finally,the dependence of measured leak-rate uncertainty on Type-B(inherent instrumental) errors was tentatively summarized.We suggest that the proposed uncertainty evaluation method be of some technological interest in leak-rate testing of spacecraft propulsion units.