Optics thermal deformation is an important factor that impacts the performance and lifetime of ion thrusters. Although some theoretical reflearch concerned with this problem was reported, its mechanism has not been fully understood. In this study, numerical investigations are performed to explain the effect of thermal deformation on the performance and lifetime of ion thrusters. The transient behavior of charged particles is calculated using a particle-in-cell simulation, while the momentum transfer collision and the charge exchange collision are calculated by means of the Monte Carlo method. Electron backstreaming restriction, perveance restriction, ions through rate, and divergence angle losses are compared and analyzed for optics deformed and undeformed. And the influence of these factors on thruster’s performance and lifetime is discussed. Results show that the ion through rate of the screen grid increases when optics begin deformed, and the thrust is slightly higher than the theoretical values predicted; the perveance threshold of the accelerator grid increases with optics haveing thermal deformation, while the crossover limit threshold is little changed, namely the thruster can be operated in conditions of a larger beam current; the electron backstreaming restriction threshold is significantly lower under a high beam current condition with optics deformed, which means that a lower accelerating gate bias is necessary to ensure thruster work. For the less obvious change of acceleratng grid current when the beam is focused, there is no moreflerosion and change of lifetime. Results provide a reflerence for the optimization design of optics and evaluation of thruster performance and lifetime.