Abstract: The performances of the monopole mass spectrometer were analyzed that it had high resolving power and focusing property. The monopole mass spectrometer whose electrode length was 100 mm and r 0=5 mm had been designed and manufactured. The resolving power and mass range of it had been measured. The researching results indicated: the mass range of the device was 223. The resolving power achieved 30 measured at 10% peak height of the N 2 +. The device could be used in residual gas analysis.