A new spectroscopic method is proposed for the characterization of optical zero-order retarders. It is demonstrated that the retardance as well as the variation of the effective fast axis of a bi-plate zero-order quarter retarder (633 nm) can be obtained with high accuracy in a broadband wavelength range by taking spectra at only three independent angular orientations of the retarder. The calibration results excellently agree with theoretical models, indicating the new method could be used as a simple and reliable way for efficient self-spectral-calibration of optical zero-order retarders.