A crystal form I of a Janus Kinase (JAK) 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 of formula (I) is provided. Also provided is a method of preparing the crystal form I of the JAK inhibitor of formula (I). The preparation method includes crystallizing any crystal form or amorphous compound solid of formula (I) in a single organic solvent to obtain the form I crystal. The crystal form I has excellent crystal stability and chemical stability. Additionally, the crystal solvent used to produce the crystal form I has low toxicity and a relatively low content of residual solvent, making the compound better suited for clinical treatment.
