The utility model relates to an indoor drainage system for preventing sewage from back flowing, which is suitable for residential buildings, office buildings, commercial buildings, guesthouses and meeting houses. The indoor drainage system comprises the structure that the height of the pipe bottom of an overflow pipe is lower than an indoor terrace; the overflow pipe is communicated with a drainage riser pipe, a downstream cross joint and the outside; a water outlet below a sanitary apparatus is connected to an outdoor drainage pipe through the downstream cross joint and a 90-degree elbow; an overflow tee-joint upper port is connected with an overflow riser pipe; flange closer plates are positioned outdoor or between staircases; a drainage elongated apical vent pipe is arranged on the upper part of the drainage riser pipe; an overflow elongated apical vent pipe is arranged on the upper part of the overflow riser pipe by an overflow cross joint which is communicated with the drainage riser pipe and the drainage elongated apical vent pipe; when certain part of the drainage riser pipe or a discharge pipe is blocked, the sewage is automatically drained from the overflow pipe and does not back flow into the room; when the flange closer plates are detached, the indoor drainage system can be dredged in short time without entering household; and remote detecting equipment can timely discover the blockage of the drainage pipeline at certain position.