The apparatus of leak detection for multi-systems with mass spectrometer is consisted of the leak detection cumulating vessel, sampling cycle system, beginning gas cumulating vessel, pressuring gas equipment, leak rate calibration equipment. The multi-systems pressured different inert gas were put into the leak detection cumulating vessel and the leak detection cumulating vessel was sealed in the air environment. The gas in the leak detection cumulating vessel was pumped into the beginning gas cumulating vessel. After the multi-systems put into the leak detection cumulating vessel, the gas in the leak detection cumulating vessel was forced to uniform. The gas of the leak detection cumulating vessel and the beginning gas cumulating vessel was alternatively analyzed by the mass spectrometer and the ion currents was accordingly got. The calibrating gas was injected into the leak detection cumulating vessel. The gas of the leak detection cumulating vessel and the beginning gas was alternatively analyzed again. The leak rates of multi-systems were calculated. The search gas of different time in leak detection cumulating vessel was comparatively measured with beginning gas stored in the beginning gas cumulating vessel. Therefore, the problem of repeat and reliability of quadpole mass spectrometer in long time was avoided. The problem of quadpole mass spectrometer calibration was avoided by injecting the quantitative calibration search gas into the leak detection cumulating vessel. The ion current of calibration search gas and the ion current of end gas were the same level by controlling the quantity of injecting the quantitative calibration search gas. The problem of the linearity of quadpole mass spectrometer also was avoided.