Provided in the present invention is a pharmaceutical composition containing a JAK kinase inhibitor or a pharmaceutically acceptable salt thereof. In particular, provided in the present invention is a pharmaceutical composition containing (3aR, 5s, 6aS)—N-(3-methoxy-1, 2, 4-thiadiazol-5-yl)-5-(methyl(7H-pyrrolo [2, 3-d] pyrimidin-4-yl) amino) hexahydrocyclopenta [c] pyrrol-2 (1H)-carboxamide, or a pharmaceutically acceptable salt thereof, and cellulose ether. The pharmaceutical composition of the present invention is characterized by a rapid dissolution rate and good stability.