Abstract: A Gaussian equation with four eigenvalues was proposed and applied to the crosslinking reactive extrusions of mPE and LDPE in a twin-screw extruder to elucidate the rules of their macroscopical mixing based on another Gaussian equation with three eigenvalues. It was found that the regression results of the Gaussian equation with four eigenvalues to the residence time distribution (RTD) curves of the crosslinking reactive extrusions of mPE and LDPE were more precise than that of the Gaussian equation with three eigenvalues，but these two kinds of Gaussian equations did not have definite physical meanings. For these reasons，we proposed a double main-flows plus single backflow cell model (revised DBFC model，named as RDBFC) according to the structural characteristics of the twin-screw extruder，which can describe the RTD curves of the crosslinking reactive extrusions of mPE and LDPE more satisfactorily because its eigenvalues all have definite physical meanings.