In this paper, a novel self-compensating phase shifter based on an equal-length unequal-width phaser is realized by inserting inductive posts to change the width of the half-mode substrate integrated waveguide (HMSIW). The phase shift of the equal-length unequal-width phaser exhibits two similar trends as frequency and width rise. Thus, the phase shift compensation can be accomplished simply by subtracting the phase shifts of these two effects. Four self-compensating phase shifters with various phase shifts are designed and simulated. And simulation results demonstrate that all four phase shifters have a return loss of better than 8dB, and a relatively flat phase shift. In particular, the 90° self-compensating phase shifter has a phase shift deviation of only 1.5° (88.5°~90°) in the frequency range of 200GHz to 240GHz.