Abstract: It is deal with a new ion optical mass spectrometer developed by the authors in 1990. The Φ_m is 130°, ε is 56°, large incident and exit angles. The radius of the magnetic field is 80 mm, and L is 132. 32mm, and with a high resolution. It can measure ~6He、~(20)Ne、~(21)Ne、~(22)He、~(36)Ar、~(38)Ar and ~(40)Ar isotopes in gas. This instrument has been used for the monitor of earthquake prodiction, and it has successfully got three examples ofearthquake.