To obtain high-quality plane waves, an diaphragm is designed to reduce the stray light caused by reflection or scattering from the tube wall of the collimator. First, a transmission model is established for the collimator, and the spots are simulated at different positions. Second, the diaphragm is simulated to analyze the influence of diaphragm thickness and cone angle on the stray light, thereby determining the appropriate parameters. Finally, the diaphragm is placed inside the collimator, and the light spot is compared with no diaphragm installed. Results indicate that without the diaphragm there is significant stray light, which overlaps the parallel light at a distance of 400 mm. After installing the diaphragm, stray light is not observed around the parallel beam, and a high-quality plane wave is obtained at 200 mm. Results indicate that stray light is effectively suppressed using the designed diaphragm. This study provides a reference scheme for suppressing stray light of the collimator during the calibration of Hartmann detectors.