Abstract: There is provided a powerless back-flow prevention apparatus comprising: a body having a sewage inlet at a front thereof, a sewage outlet at a rear thereof, and a length-wise sewage channel there between; a door pivotally coupled to the body at the rear thereof to open or close the outlet, wherein the door comprises: a number of floating members arranged spacedly with each other; and a guide disposed on the floating members to pivot upwards via the floating force to close the outlet, and to pivot downwards via a gravity in an absence of the sewage back-flow to open the outlet, and, then to guide the sewage from the outlet.