A double-slit type monitor will be used in the emittance measurements at the exit of the first tank of the drift tube linac in the China Spallation Neutron Source where the H−ion energy is 21.67 MeV. The rapid energy deposition in the first slit may generate a rapid temperature rise and high thermal stress, which may damage the slit plate. This article presents detailed thermo-mechanical studies for the slit by Monte Carlo program and finite element analysis. The existence of Bragg peak of the stopping power introduces a thermal spike and stress peak in the slit plate, and both the peak values reach a quasi-steady-state after several beam pulses, moreover, they show different variation trends with changing the beam repetition rate and pulse width. The equilibrium peak temperature and stress are about 1605 K and 78 MPa, respectively, at beam pulse width of 100 μs and repetition rate of 5 Hz when the beam centered at the edge of slit, which does not exceed the corresponding threshold value of graphite, however, high cycling stress may bring about fatigue damage, so the beam parameters should be selected at suitable values according to the demands of actual measurements.