The present invention relates to a crystal form of JAK kinase inhibitor bisulfate and a preparation method thereof. In particular, the present invention relates to a type II crystal of (3 aR, 5 s, 6 aS ) -N- (3-methoxyl-1, 2, 4-thiadiazole-5-yl)-5-(methyl(7 H- pyrrolo[2, 3- d ]pyrimi dine-4-yl)amino)hexahydrocyclopenta[ c ]pyrrole-2(1 H )-formamide bisulfate and a preparation method thereof. The preparation method comprises crystallizing any crystal forms or amorphous compound of formula (I) solid in a single organic solvent or a mixed organic solvent to obtain a type II crystal form of a compound of formula (I). The type II crystal form of compound of formula (I) obtained by the present invention has a good crystal stability and chemical stability, and the used crystal solvent has a low toxicity and residue, which can be better used in clinical treatment.