The invention discloses a horizontal mold for integrated prefabrication of a pier column and a cover beam and a construction method thereof. The prefabrication mold comprises a pier column formwork structure, a cover beam formwork structure and a mold bottom frame, the mold bottom frame is T-shaped and comprises a transverse edge and a longitudinal edge; the cover beam formwork structure is arranged along the transverse edge of the mold bottom frame, the pier column formwork structure is arranged along the longitudinal edge of the mold bottom frame, the pier column formwork structure communicates with the cover beam formwork structure, the horizontal section of the cover beam formwork structure is rectangular or trapezoid, and a slurry passing opening is formed at the position, connected with the pier column formwork structure, of a side mold of the cover beam formwork structure; and the pier column formwork structure is cylindrical, and comprises a pier column bottom mold, a pier column top mold and a plugging plate, the cross section of the pier column bottom mold is in a semi-arc shape, the pier column top mold is connected to the top of the pier column bottom mold, and first pouring holes are formed in the tops of pier column top mold units. According to the mold and the construction method, the technical problems that for a traditional mode of casting the pier column and the cover beam in situ correspondingly, the construction period is long, the safety is poor, the construction quality is not easy to guarantee, and the environmental protection pressure is large are solved.