Abstract: Spacecraft leak testing is divided into component testing and system testing. System leak test mostly take the method of non-vacuum accumulation leak testing by helium mass spectroscopy. Due to the uneven distribution of helium in the accumulation chamber and the unstable readings of helium mass spectrometer leak detector， the initial and final leakage rate values may not be read accurately which may result in the measurement error. In this paper， a non-vacuum accumulation leak testing by helium mass spectroscopy based on least square method is introduced. In the process of helium gas accumulation， read the helium gas density in the accumulation chamber at regular intervals and calculate the helium gas density charge rate using least square method. Then， the leakage rate of the object under test is given. A large number of test results show that， compared with the traditional methods， this method can effectively avoid the measurement error caused by single testing and the results have a higher credibility.