Crystal forms I and II of tasimelteon. Crystal form I: an X-ray powder diffraction spectrum using CuKα radiation and represented by a 2θ angle has diffraction peaks at least at 7.2°±0.2°, 7.9°±0.2°, 10.6°±0.2°, 14.4°±0.2°, 15.9°±0.2°, 17.3°±0.2°, 21.0°±0.2°, 23.2°±0.2°, and 24.4°±0.2°. Crystal form II: an X-ray powder diffraction spectrum using CuKα radiation and represented by a 2θ angle has diffraction peaks at least at 6.8°±0.2°, 12.1°±0.2°, 12.5°±0.2°, 13.1°±0.2°, 13.6°±0.2°, 13.8°±0.2°, 15.8°±0.2°, 17.0°±0.2°, 18.4°±0.2°, 20.8°±0.2°, 24.2°±0.2°, and 24.4°±0.2°. Crystal forms I and II of tasimelteon have advantages of excellent physicochemical property, good stability and solubility, and simple operation for preparation.