The invention discloses a wavelength correction type multi-beam step plane reflecting mirror laser interferometer and a wavelength correction method. The wavelength correction type multi-beam step plane reflecting mirror laser interferometer comprises a laser source, a beam splitter, a fixed reflecting mirror, a measurement reflecting mirror device and a photoelectric detector group. The laser source comprises n (n>=2) parallel laser beams. The photoelectric detector group comprises n photoelectric detectors. The measurement reflecting mirror device comprises a measurement reflecting mirror and a precision displacement device. The reflecting surfaces of the fixed reflecting mirror are n step planes. Distance between two adjacent reflecting planes is lambda/2n+klambda/2 (k is a natural number). Each beam of laser is divided into two beams through the beam splitter, wherein one beam transmits through the beam splitter through reflection of the fixed reflecting mirror to reach the photoelectric detectors, and the other beam of laser is also incident to the photoelectric detectors through reflection of the measurement reflecting mirror and the beam splitter in turn. The laser interference phenomenon generated by the laser interferometer is related to laser wavelength and also related to the height difference values of the step type reflecting planes. Displacement of which precision reaches the level of lambda/2n can be detected by the photoelectric detector group, and environmental equivalent wavelength can be acquired by adopting the wavelength correction method so that measurement precision can be substantially enhanced.