Calculating the polarization properties and revealing the mechanism or the sources of depolarization properties of tissue are all essential for their successful applications. In this work we report a method of calculating differential polarization parameters directly from measured macroscopic Mueller matrices and revealing the mechanism or the source of depolarization properties of tissue. First a measured Mueller matrix is expressed as a sum of up to four Mueller-Jones matrices. The derivatives of both sides of the equation are carried out to find both the non-depolarizing and depolarizing differential polarization parameters by using the differential Mueller matrix equation. The explicit formulas for each Mueller-Jones matrix are then obtained in terms of the polarization parameters of differential Mueller matrix of the general Mueller matrix. Our method has the advantages of being capable of identifying measurement errors contained in the measured Mueller matrices, separating non-depolarizing and depolarizing differential polarization parameters, and revealing contributions of depolarizing differential polarization parameters to depolarization behaviors of the medium. The experimental results are presented to demonstrate the potential applications of our method.