Abstract: The calculation errors of particle-in-cell (PIC) method at the conformal boundary are analyzed, especially for the conformal boundary at the THz cyclotron oscillator. The condition of stability and two general treatment methods for the conformal grid are put forward. And the stability and accuracy of these two conformal treatment methods are analyzed. Based on these analysis, an adaptive conformal cell subdivision method is developed. With this method, the problems of the limitation of the conformal grid are solved. The method is implemented on the electromagnetic particle simulation software CHIPIC and is verified by simulating a cylindrical waveguide. Finally, using this method, the THz cyclotron maser with TE26 and the quasi optical cavity cyclotron oscillator with HE06 are simulated successfully.