The invention relates to a sewage purifying pretreatment apparatus. The sewage purifying pretreatment apparatus comprises a casing, a filter screen plate is horizontally arranged in the casing, the slide blocks are fixedly arranged at left and right ends of the filter screen plate, the sliding grooves are vertically arranged on the left and right sidewalls of the casing, the slide blocks are arranged in the sliding grooves in a sliding mode; the fixed plates are symmetrically arranged on the left and right inner walls of the casing, a buffer spring is arranged between the fixed plate and the filter screen plate; a pipe sleeve is sleeved on a jet pipe, the pipe sleeve is fixed at the bottom of the fixed plate, a flushing nozzle is arranged at the upper end of the jet pipe; a connecting rod is arranged at the left end of a telescopic hydraulic cylinder, a brush plate is fixedly arranged at the left end of the connecting rod, and a hairbrush is arranged at the bottom of the brush plate. The filter screen plate performs preliminary filtering on sewage, the foreign materials attached on the filter screen plate can be cleaned by the brush plate, the meshes on the filter screen plate can be effectively dredged, the flushing nozzle is obliquely upward for spraying water to the filter screen plate, so that the meshes in the filter screen plate can be flushed, the mesh obstruction can be effectively prevented, and the sewage pretreatment effect is greatly increased.