Transmissivity anomaly and polarization response of grating spectrometers have been discussed. It is shown that these characteristics of the spectrometers will have great influences on spectroscopic measurements. In order to diminish the influence, a 90° optical rotator was used to measure Raman polarization spectrum. For some polarization directions, signal intensity obtained could be increased by several times, and it might be one time stronger than that measured with a depolarizer. Depolarization ratio of Raman spectrum peaks of CCl4 was measured with the result being the same as that measured by other methods.