Provided is an I-type crystal of a JAK (Janus Kinase) inhibitor (3aR,5s,6aS)-N-(3-methoxyl-1,2,4-thiadiazole-5-yl)-5-(methyl(7H-pyrrolo[2,3-d]pyrimidine-4-yl)amino)hexahydrocyclopenta[c]pyrrole-2(1H)-formamide bisulfate represented by formula (I), and a preparation method therefor. The preparation method comprises crystallizing any crystal form of or amorphous compound solid of formula (I) in a single organic solvent to obtain the I-type crystal； this crystal has excellent crystal stability and chemical stability, and the crystal solvent used is low in toxicity and residues, making the compound better applicable to clinical treatment.
