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 (3aR,5s,6aS)-N-(3-methoxy-1,2,4-thiadiazole-5-yl)-5-(methyl(7H-pyrrolo[2,3-d]pyrimidine-4-yl)amino)hexahydrocyclopenta[c]pyrrole-2(1H)-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.
