The invention discloses a process for casting seal rainproof complex flanges. The complex flanges comprise first flanges and second flanges. Each first flange and the corresponding second flange are butted to each other in the left-right direction, each first flange comprises a first flange body and a protective cover, at least four first fixing holes are uniformly formed in each first flange body, at least four oil filler holes are further uniformly formed in each first flange body, first lugs are further respectively uniformly arranged on the surface of each first flange body, and first grooves are further respectively uniformly formed in the surface of each first flange body; each second flange comprises a second flange body, at least four second fixing holes are uniformly formed in each second flange body, second lugs are further respectively uniformly arranged on each second flange body, and second grooves are further respectively uniformly formed in each second flange body. The process has the advantages that the process is simple and feasible, the seal rainproof complex flanges cast by the aid of the process are low in casting cost, simple in structure, high in connectivity and good in sealing performance, lubrication can be facilitated when the seal rainproof complex flanges are used, rainproof effects can be realized, the service lives of the seal rainproof complex flanges can be prolonged, and the cost can be reduced.