The invention discloses a 4-chloro-7H-pyrrolo[2,3-d]pyrimidine synthetic method. A compound is an important intermediate for synthesizing ruxolitinib and tofacitinib as a JAK inhibitor for treating rheumatoid arthritis. The 4-chloro-7H-pyrrolo[2,3-d]pyrimidine synthetic method comprises the following steps of by taking a compound I (4,6-dichloro-5-allyl pyrimidine) as a starting material, performing oxidation reaction on the compound I and ozone to produce a compound II; then performing nucleophilic substitution reaction on the compound II and triethyl orthoformate to produce a compound III; then performing nucleophilic substitution reaction on the compound III and ammonia gas to produce a compound IV; and finally, performing ring closing on the compound IV self in an acid environment to produce a compound V, i.e., 4-chloro-7H-pyrrolo[2,3-d]pyrimidine, wherein a synthetic route is shown as the following formula (described in the description). The synthetic method disclosed by the invention is cheap and available in raw materials, simple and short in synthetic route, low in cost, high in yield and easy in industrial production.