The invention provides a mounting structure and method of a prefabricated bay window. The mounting structure comprises the prefabricated bay window, a prefabricated outer wall, a reserved hole, a positioning pre-embedded piece, a connecting piece, heat insulating layers and protective layers. The mounting method comprises the following steps that a, the reserved hole is formed in the prefabricatedbay window, and the corresponding heat insulating layer and protective layer are arranged outside the prefabricated bay window; b, the positioning pre-embedded piece is arranged on a bay window reserved opening, and the corresponding heat insulating layer and protective layer are arranged outside the prefabricated outer wall; c, the prefabricated bay window is embedded into the bay window reserved opening, and the heat insulating layer and the protective layer of the prefabricated bay window as well as the heat insulating layer and the protective layer of the prefabricated outer wall are spliced orderly; and d, the connecting piece penetrates through the reserved hole to be connected with the positioning pre-embedded piece, fixing of the prefabricated bay window on the prefabricated outerwall is realized, and sealants are injected to all connecting gaps. The mounting structure and method of the prefabricated bay window have the advantages that the prefabricated bay window is integrally prefabricated, the mounting structure is simple, and the construction period of the bay window is effectively shortened.