Depolarization is a main paramenter of the depolarizer. In order to improve the performance of the double plate rotation depolarizer, the average Mueller matrix is used to derive the theory expression of depolarization. The relationship between depolarization-polarization and related parameters is propased by using the average Mueller matrix and Stokes vector. Through the first order Bessel function solution, a satisfactory explanation of the depolarization for the depolarizer in theory is gained. Depolarization is related to some parameters such as the wavelength, the beam diameters the structural angle etc. A digital simulation is performed based on Matlab software. The theory results show that the shorter wavelength depolarizers is easier, depolarization effects improve when inereasing pupil radius or angle structure. Theoretical results meet well with the experiment. The discussions can supply theoretical basis for the design, production and use of the depolarizer.