A method for constructing a prefabricated bathroom comprising a bottom plate unit, multiple wall units and a top plate unit that are connected to each other in a securing manner, wherein one of the wall units is a structural wall, which is provided, at its top portion, with a recessed top region reserved for later pouring, the recessed top region being provided therein with top exposed reserve bars, said structural wall is configured as a full-prefabricated structural wall body at its middle portion, and is provided, at its bottom portion, with a recessed bottom region reserved for later pouring, the recessed bottom region being provided therein with bottom exposed reserve bars, and the top exposed reserve bars of the prefabricated bathroom of a floor are inserted into the recessed bottom region of the prefabricated bathroom of a next floor, and connected with the bottom exposed reserve bars thereof, so that the prefabricated bathroom of said floor can be connected with the prefabricated bathroom of said next floor through pouring concrete in the recessed top region and the recessed bottom region, the method comprising the following steps: Step A: building the bottom plate unit and the lower portions of said multiple wall units with a bottom plate steel mold and a vertical steel mold, bundling the bars of the bottom plate unit and the wall units, and pouring concrete; Step B: forming the recessed bottom region reserved for later pouring at a bottom part of the structural wall, and providing exposed bars in the recessed bottom region; Step C: building the upper portions of said multiple wall units and the top plate unit with a vertical steel mold of the wall units and a top mold arranged at the top of the top plate unit, bundling the bars of the top plate unit and the wall units, and pouring concrete; and Step D: forming the recessed top region reserved for later pouring at a top part of the structural wall, and providing exposed bars in the recessed top region. wherein one of the wall units is a structural wall, which is provided, at its top portion, with a recessed top region reserved for later pouring, the recessed top region being provided therein with top exposed reserve bars, said structural wall is configured as a full-prefabricated structural wall body at its middle portion, and is provided, at its bottom portion, with a recessed bottom region reserved for later pouring, the recessed bottom region being provided therein with bottom exposed reserve bars, and the top exposed reserve bars of the prefabricated bathroom of a floor are inserted into the recessed bottom region of the prefabricated bathroom of a next floor, and connected with the bottom exposed reserve bars thereof, so that the prefabricated bathroom of said floor can be connected with the prefabricated bathroom of said next floor through pouring concrete in the recessed top region and the recessed bottom region, the method comprising the following steps: Step A: building the bottom plate unit and the lower portions of said multiple wall units with a bottom plate steel mold and a vertical steel mold, bundling the bars of the bottom plate unit and the wall units, and pouring concrete; Step B: forming the recessed bottom region reserved for later pouring at a bottom part of the structural wall, and providing exposed bars in the recessed bottom region; Step C: building the upper portions of said multiple wall units and the top plate unit with a vertical steel mold of the wall units and a top mold arranged at the top of the top plate unit, bundling the bars of the top plate unit and the wall units, and pouring concrete; and Step D: forming the recessed top region reserved for later pouring at a top part of the structural wall, and providing exposed bars in the recessed top region. Step A: building the bottom plate unit and the lower portions of said multiple wall units with a bottom plate steel mold and a vertical steel mold, bundling the bars of the bottom plate unit and the wall units, and pouring concrete; Step B: forming the recessed bottom region reserved for later pouring at a bottom part of the structural wall, and providing exposed bars in the recessed bottom region; Step C: building the upper portions of said multiple wall units and the top plate unit with a vertical steel mold of the wall units and a top mold arranged at the top of the top plate unit, bundling the bars of the top plate unit and the wall units, and pouring concrete; and Step D: forming the recessed top region reserved for later pouring at a top part of the structural wall, and providing exposed bars in the recessed top region.