Inductively coupled plasma mass spectrometer (ICP-MS) is one of the most powerful analytical methods for trace elements due to its high sensitivity and low detection limit. In the design of ICP-MS instruments, effectively introducing the ion beam of the analyte into the post-mass analyzer system is a major challenge. To achieve efficient transmission of ion beams, the ion beam should have a small angle divergence and energy divergence after entering the deflection lens, and be emitted as parallel as possible. That is, the velocity in the vertical direction should be as low as possible. At the same time, to remove neutral particles and photons to the maximum extent and reduce background noise, the vertical deflection displacement should be made larger. This paper draws on the theory of electrostatic fields in electron optics, starting from the motion process of ions in an electrostatic field, and theoretically derives and establishes a model for the motion process of ions in a polarized suspended off-axis deflection lens. Then, using SIMION software, the motion of Li, In, and U ion beams representing low, medium, and high mass numbers in this deflection lens system is simulated and analyzed. The key electric field parameters acceleration voltage Ua, deflection voltage U1, deflection voltage U2, and mechanical parameter L2 in theoretical analysis are examined for their effects on ion beam transmission, including ion deflection displacement sy, vertical velocity v′y at the exit hole, and ion pass rate. Combined with simulation results and design, the effects are investigated. Based on the simulation results and design requirements, this article presents the key parameter values and size optimization design scheme for the off-axis deflection lens. According to the optimized design scheme provided by software simulation, three sizes of deflection lens systems were applied to ICP-MS instruments for comparative testing of background noise and sensitivity to verify the reliability of the simulation. The experimental results showed that the background noise and sensitivity were significantly improved with the optimized structure, especially for low-mass numbers. The background noise was reduced by about 2 times and the sensitivity was increased by about 4 times. Although there may be some errors between simulation and actual results, the overall trend is correct. It can guide the structural design and electrical parameter optimization of deflection lens systems in practice.