External drainage system for water collecting basin of integral bathroom and construction method of external drainage system

The invention belongs to the technical field of assembly type toilets, and relates to an external drainage system of a water collecting basin of a whole bathroom and a construction method of the external drainage system. The external drainage system at least comprises a sewage pipe, a waste water pipe, a water collecting basin and a drainage collector, and sewage from the sewage pipe and waste water from the waste water pipe are collected and discharged; the water collecting basin comprises a bottom basin which is provided with a basin frame used for defining a floor laying area and a floor, the bottom basin is laid in the floor laying area, the side face of the basin frame is provided with a drainage port used for being communicated with a waste water pipe, and the drainage collector is installed on one side of the basin frame. Due to the fact that the drainage collector is installed on one side of the basin frame of the water collecting basin, a drainage pipeline does not need to be arranged below the botto