The invention discloses a fused pyrimidine compound, and an intermediate, a preparation method, a composition and an application of. The invention provides a fused pyrimidine compound represented by formula I, and a tautomer, an enantiomer, a diastereoisomer, a pharmaceutically acceptable salt, a metabolite, a metabolism precursor or a drug precursor thereof. The invention also provides an application of the fused pyrimidine compound represented by formula I, and the tautomer, the enantiomer, the diastereoisomer, the pharmaceutically acceptable salt, the metabolite, the metabolism precursor or the drug precursor thereof in the preparation of drugs for preventing, alleviating or treating one or more of immune system diseases, autoimmune diseases, cell proliferation diseases, allergic diseases and cardiovascular system diseases. The compound has very strong inhibition effect on Janus kinase, FGFR kinase, FLT3 kinase and Src family kinase.