A novel quasi monochromatic depolarizerimproved Lyot depolarizer was fabricated and analysed in detail. Three conditions for effective depolarization were concluded: 1) retardance of the depolarizer should be large enough; 2) the optical pupil or the wedge angle of the crystal should be large enough; 3) angle between two crystal′s optical axes should be 45°. The experimental results indicated that, for arbitrary incident quasi monochromatic polarized light (±10 nm) ranging of 400～800 nm, the average residual polarization degree is less than 1.2% after depolarized by this depolarizer.