Provided are a crystalline form of an androgen receptor inhibitor and a preparation method thereof. In particular, provided are type I crystal of (S)-4-(3-(4-(2, 3-dihydroxypropoxy)phenyl)-4, 4-dimethyl-5-carbonyl-2-thioimidazolin-1-yl)-2-(trifluoromethyl)benzonitrile (a compound of formula (I)) and a preparation method thereof, wherein the method comprises : 1) any crystal form or amorphous form of the compound as shown by formula (I) is added to an appropriate amount of an organic solvent, heated and dissolved until clear, following by being cooled down and crystallized, wherein the organic solvent is selected from any one of esters, ketones, nitriles and ethers which have 5 or less carbon atoms, or a mixed solvent thereof; and 2) the crystal is filtered, washed and dried. The obtained type I crystal of the compound of formula (I) has a good chemical stability and crystal form stability, and the crystallization solvent used has a low toxicity and residue, and can be better used for clinical treatment.