Abstract: Ion optics is a critical aspect in the designing of a mass spectrometer. For improving the abundance sensitivity, a new magnetic-electric-quadrupole (MEQ) tandem mass spectrometer was designed and manufactured. In this MEQ tandem mass spectrometer, the lens in the last stage is quite different from that of normal double focus mass spectrometers or quadrupole mass spectrometers. In this paper, the ion optics design of the instrument is presented, together with detailed design of the lens neighboring the quadrupole, the numerical calculating result of the ion trajectory in these lenses and the quadrupole with the help of SIMION 7.0 software. The transferring efficiency for the last stage is evaluated, and tested to be around 9% in experiment, which is in accordance with the calculated result.