Polarization intermodulated. excitation (POLINEX) spectroscopy is a new nonlinear high resolution speotroHCOpy. It combines polarization spectroscopy with intermodulated fluorescence spectroscopy. So the POLINEX spectroscopy has some advantages of them. In author's former paper, we had used the rate equation approach to analyze how the Doppler background caused by atomic collisions was eliminated and how the Loren-tzian line shape was kept. In this paper, we use the semiolassioal theory of Lamb to analyze the POLINEX spectroscopy further more. This method is more reasonable. Considering laser beam's polarization^ energy level's degeneracy and collision cross relaxation, by means of the equation of motion of density matrix, we have calculated the polarizability of the atomic system submitted to laser beams and J2 to the third order χ(3). Then，from %m we have evaluated, nonlinear components of the absorption ooeffioient a. So we can obtain the strength and shape of the intermodulated signal in POLINEX spectroscopy. The conclusion is in conformity with the experimental results. Moreover, it predicts the pressure shift and pressure broadening which can't be predicted by the rate equation approaoh.