The invention discloses a calibration method for peak retardation of a photoelastic modulator. A calibration device used by the calibration method comprises two parts of an optical measurement module and a signal processing module, wherein the optical measurement module is composed of a laser, a polarized beam splitter, an eighth-wave plate, the photoelastic modulator and a reflector, and modulation on the polarization state of incident light is completed altogether; and the signal processing module is composed of a photoelectric detector, a signal regulator, a phase-locked amplifier, the photoelastic modulation driver and a computer, and the peak retardation of the photoelastic modulator is obtained. The calibration method has high sensitivity when the photoelastic modulator works in a small-angle modulation state; as laser beams pass through the photoelastic modulator for two times successively, the calibration precision is improved by two times that of the traditional method; the calibration result is not influenced by incident light intensity fluctuation, the offset between the actual peak retardation of the photoelastic modulator caused by external environment and a set value can be corrected, and the precision of an atom spin precession detection result in an atom magnetometer can be improved.