Abstract: In order to grasp the angle and energy responses of allyl diglycol carbonate (CR-39) detector to incident alpha particles, a 241Am irradiation system which was allowed to adjust the incident angles and energies of alpha particles was set up. Through the experiment and comparison of the track densities in different irradiation conditions, it was found that the CR-39 detector is sensitive to those alpha particles with incident energies larger than 0.5 MeV and incident angles less than about 70°. The results are helpful for the optimizing design of diffusion chamber for airborne radon measurements with CR-39 detectors.