The invention discloses a rear floor assembly structure for a plug-in hybrid electric vehicle, which comprises a rear-floor front part and a rear-floor rear part, wherein a left side longitudinal beam and a right side longitudinal beam extending to the downside of the rear-floor rear part are respectively arranged on the two sides of the downside of the rear-floor front part, a cross beam is connected between the left side longitudinal beam and the right side longitudinal beam below the rear-floor front part and is connected with a fuel tank and an exhaust pipe muffler, a battery compartment is arranged on the rear-floor rear part between the left side longitudinal beam and the right side longitudinal beam, a battery pack is hung above the battery compartment, the front end of the battery pack is connected with a battery cooling system, and the battery cooling system is fixed above the rear-floor front part. According to the rear floor assembly structure, a mode of directly loading floors in the conventional thinking is abandoned, but the battery pack is hung above a rear floor at the bottom of a suitcase and loaded on the side longitudinal beams, so that the strength is ensured, and meanwhile a stable fixation state is formed; and through reasonable partitioning, spaces above and below the rear floor are fully used, so that the function components such as the battery pack and the cooling system thereof, the fuel tank and the exhaust pipe muffler are reasonably arranged on a vehicle body.