The invention belongs to the technical field of pharmaceutical synthesis, in particular to a preparation process of a Xeljanz intermediate. The preparation process comprises the following steps: 1) adding ethyl cyanoacetate into n-butyl alcohol, and uniformly stirring; 2) dropwise adding DBU into a solution prepared in the step 1) at a temperature of or below 30 DEG C; 3) adding N-methyl-N-((3R,4R)-4-methylpiperidine-3-yl)-7H-pyrrolo[2,3-D]pyrimidine-4-amine into a solution prepared in the step 2) in batches for reacting; 4) dropwise adding water into a system reacting completely in the step 3), stirring, filtering and washing to obtain the Xeljanz intermediate, namely, (3R,4R)-4-methyl-3-(methyl-1H-pyrrolo[2,3-d]pyridine-4-amino)-beta-oxy-1-piperidine propionitrile. The intermediate prepared by the invention has higher yield and purity, and the decrease of the purity of the intermediate in expanded production is less.