Laser plumbing instrument and beam splitting device thereof

The invention discloses a beam splitting device of a laser plumbing instrument. The beam splitting device is characterized by comprising an incident surface, wherein a transmission region is arranged in the center of the incident surface; four reflection surfaces are arranged on the opposite sides of the incident surface and beside the transmission region; four emergent surfaces are in correspondence to the four reflection surfaces; the angles of the reflection surfaces are arranged to enable incident beams to be reflected to the emergent surfaces for emergence, emergent beams are vertical to the incident beams, and the included angle between every two adjacent emergent beams is 90 degrees. According to the beam splitting device with through holes and the four reflection surfaces, a beam is split into five beams through an optical element, so that plumbing of five points can be realized by only a laser tube, the size of the plumbing instrument is reduced, and the cost is lowered.