Abstract: Based on the Lyot depolarizer, a new type of depolarizer for monochromatic light was designed. This depolarizer consisted of two quarter-wave plates with slow-axis angle of 45. By analyzing the superimposition of transmitted light, it was found that the incident linearly polarized light of arbitrary azimuth angle of vibration could be converted to circularly polarized light with stable intensity, and the formula of depolarization degree was derived. For the waveband of 633 nm, two quarter-wave plates were selected purposely to make the sample whose depolarization degree was tested. Experiment results indicate that the depolarization degree was over 94% and it is over 97% as the incident angle is adjusted slightly.