Abstract: In heavy-ion cancer therapy, Compton camera is a promising tool for online monitoring of the range of ions. Compton camera uses crystal detectors to determine the positions and deposited energies of the \gamma-rays. This will further introduce the errors and affect the actual imaging resolution of the Compton camera, due to the involved Doppler broadening effects influencing the image resolution. This work simulates the angular resolution measure originating from the Doppler broadening effects with Geant4 toolkit for 150 and 511 keV gamma, respectively, in different crystal materials. After optimizing the back-projection algorithm and improving the voxel in imaging interspace, the image resolution can be achieved better than 1.0 mm. An approximate formula is also being proposed to evaluate the image resolution based on the angular resolution measure.