In order to study the depolarization performance for monochromatic light of improved Lyot depolarizer, the method of superposition of multiple beam is proposed. The formula for depolarization degree D that is the function of vibration azimuth angle (VAA) and total retardation is deduced. It is shown by theoretical analysis that D is ideal for any VAA when retardation δ=（N+ 1/2）π (N is integer) on the basis of large enough wedge angle. And when δ=Nπ, VAA makes greatest impact on D. When δ is assigned to the other values, the impact on D made by VAA is between the former two. Using 405 nm diode laser whose beam is expanded and collimated, experiments to measure D of sample whose wedge angle is 6° are conducted. The theoretical results are well verified by experiments. D is over 98.8% when total retardation δ is nearly (N+ 1/2)π by changing incident angle.