The invention relates to a prefabricated bay-window and a construction method thereof, and belongs to the technical field of fabricated buildings. The prefabricated bay-window comprises a mounting frame, a bottom plate and two side plates are fixedly connected to the mounting frame along the edge of a window opening, the two side plates are located on the two sides of the window opening correspondingly, the bottom plate is perpendicular to the side plates, the mounting frame, the bottom plate and the side plates are integrally formed, and a connecting steel bar is reserved on the mounting frame; and a mounting groove is formed in the mounting frame, the mounting groove and the side plates are located on the same side of the mounting frame, the mounting groove is located on the sides, awayfrom the bottom plate, of the side plates, and the side, away from the mounting frame, of the bottom plate exceeds the edge of the side plates. The invention further provides the construction method,and the construction method comprises the steps of reservation of a mounting opening, mounting of the prefabricated bay-window and pouring. The prefabricated bay-window has the effect that the daylight effect is highly adjusted, and the construction method is simple and quick.