A new method to calibrate accurately a photoelastic modulator is proposed to compensate for the shortcomings of current calibration methods. Firstly, the photoelastic modulator is calibrated roughly to locate its peak retardation around 1.841 rad using the fundamental component's maximum of the detection signal in an optical system for calibration which is composed of polarizer, waveplate, photoelastic modulator and analyzer. Then the waveplate is taken away to form an optical system for accurate calibration. Before and after the analyzer is rotated, the direct current and secondary harmonic components of the detection signals are obtained to calculate the peak retardation of the photoelastic modulator. The accurate calibration for the photoelastic modulator is realized and its precision is independent of the incident intensity and the circuit parameters. The calibration method was verified by experiments. The calibrated error of the method is less than 0.7%. The peak retardation of the photoelastic modulator is small in the calibration, so the photoelastic modulator can be calibrated in the long-wave band of its spectral range using this calibration method.