In order to study the depolarization properties of an improved Lyot depolarizer with monochromatic light, the theory of multi-beam superposition is adopted. The dependences of degree of depolarization (D) on vibration azimuth angle (VAA) and total retardation (TR) are analyzed. The results show that D is ideal for any VAA when TR is δ=(N+1/2)π (N is an integer) on the basis of wedge angle large enough. And when δ=Nπ, VAA makes the most significant impact on D. When δ is assigned to the other values, the impact on D made by VAA is between the former two. Using a 405 nm semiconductor laser, experiments for measuring D of the sample with wedge angle of 6° are conducted. The theoretical results are well verified by experiments. D is over 98.8% when TR is nearly δ=(N+1/2)π by changing the incident angle.