The present invention relates to a passive microwave sounder for a satellite, having a fixed reflection plate. The passive microwave sounder for a satellite, having a fixed reflection plate includes: a motor 100 including a first rotary shaft 110 formed to extend in a progressing direction of a satellite; a first rotating reflection plate 200 forming a predetermined angle with respect to the ground surface of a nadir direction and having a first one-side surface 210 and a first other-side surface 220, the center of the first one-side surface 210 being coupled to and rotating with the first rotary shaft 110, such that the first one-side surface 210 and the first other-side surface 220 alternately face the ground surface, and the second other-side surface 220 reflecting incident electromagnetic waves; an auxiliary reflection part 300 reflecting the electromagnetic waves incident from the first other-side surface 220 to a predetermined position; a reception part 400 receiving the electromagnetic waves reflected from the auxiliary reflection part 300; and a fixed reflection plate 500 fixed above the first rotating reflection plate 200 at a predetermined angle with the ground surface and reflecting the electromagnetic waves to the first one-side surface 210 or the first other-side surface 220.