The invention discloses a rain-sewage diversion method for garbage first-layer landfill. The method includes when first-layer landfill is performed on a landfill partition of a landfill, laying a layer of mulch on half or all of the landfill partition to ensure rain not to permeate into sewage in the landfill partition so as to retain the rain on the mulch to greatest extent; in other landfill partitions where landfill is not performed, discharging collected rain into a rain pipe duct through a changeover valve. By combing use of a cofferdam, rain-sewage diversion of first-layer landfill of the landfill can be realized effectively, and the rain on the mulch can be discharged timely, so that generation quantity of percolate in a landfill reservoir area is effectively reduced.