Angle errors of high-order retarders will decrease the accuracy of a channeled spectropolarimeter. This paper presents an easily implemented and widely applicable method for reducing the effects of the angle errors. First, we theoretically derive a modified reconstruction model to express and analyze the effects of the angle errors. Based on the modified reconstruction model and current reference beam calibration technique, we put forward the modified reference beam calibration technique to reduce the effects of the angle errors. This method can calculate the angle errors by employing the amplitude terms, which have been ignored in the results of the current reference beam calibration. The effectiveness of the presented method is verified by numerical simulations, which show that the demodulated deviations of polarization parameters have been reduced by one order of magnitude. Experiments are further implemented to validate the proposed method. The convenience and wide applicability of the presented method make it suitable for regular correction of the instrument, especially for the case on track.