Abstract: Some microwave chemical reactors of high temperature and high pressure use circular ceramic as dielectric coupling window. In this paper, based on calculation of finite-element method (FEM), the distributions of thermal and mechanical stresses were computed with different pressures in the reactor and input microwave power, and these two stresses were analysed and compared. Simulations indicate that the distribution of thermal and mechanical stresses on the dielectric coupling window and the window’s deformation are changed greatly with the pressure and power. And the primary stress on that window is different in different conditions. The primary stress has important reference value in the design of dielectric coupling window in high temperature and high pressure chemical reactors.