A crystalline form of an androgen receptor inhibitor and its preparation method are provided. In particular, provided are crystal form I of (S)-4-(3-(4-(2,3-dihydroxypropoxy)phenyl)-4,4-dimethyl-5-carbonyl-2-thioimidazolin-2-yl)-2-(trifluoromethyl)benzonitrile (a compound of formula (I)) and its preparation method. The method includes (a) adding any crystal form or amorphous form of the compound of formula (I) to an organic solvent to obtain a solution and heating the solution until it is clear, followed by cooling the solution to precipitate a crystal； and (b) filtering, washing, and drying the crystal. The obtained crystal form I of the compound of formula (I) has good chemical stability and crystal form stability, and the crystallization solvent used has low toxicity and residue, and is thus better for use in clinical treatment.
