The invention provides an A crystal form of baricitinib. Positions where 2-theta equals to 12.46, 13.921, 14.94, 15.359, 16.26, 16.639, 17.36, 19.08, 20.321, 21.961, 22.381, 24.118, 25.42, 27.441, 28.381, 29.321, 29.799, 32.675, 33.14, 33.563, 33.923 and 41.6 have diffraction peaks, wherein the error range of the 2-theta value is + / - 0.2. The provided A crystal form of the baricitinib has good high-temperature stability, high-humidity stability and illumination stability, can be applied to the medicine treating or preventing diseases related to JAK, has better bioavailability, and provided qualitative and quantitative information has very important significance for further studying the curative effect of the solid drugs.