The present application relates to a crystal of a tricyclic compound, in particular to a crystal of (S)-8-(4,4-difluorocyclohexyl)-8H-thieno[3,4]pyrrolo[1,5-a]imidazole, a preparation method therefor, a crystal composition and a pharmaceutical composition thereof and the use thereof. The X-ray powder diffraction spectrum of the crystal of the compound of formula I in the present application, represented by a 2θ value, has a diffraction peak at about 11.49°, 15.05°, 20.14°, 21.53° or 21.79°. The crystal of the compound of formula I in the present application has excellent physical properties, high safety and metabolic stability, and has a good inhibitory effect on IDO and a high medicinal value.