Polarization intermodulated excitation (POLINEX) spectroscopy is a new developing Doppler-free spectroscopic technique which combines the advantages of the polarization spectroscopy and the intermodulated fluorescence speotroscopy. In this technique, a tunable laser beam is circular polarized first and then divided into two parts of nearly same intensities. Two rotating polarizers of different modulated frequencies are used to change the directions of these linear polarized beams periodically with time. When they interact with the atoms, the fluorescence signal are detected via a lock-in amplifier. The POLINEX spectroscopy can effectively eliminate the Doppler-broadened pedestal due to the velocity-changing collisions among atoms as the polarization-modulation is used instead of the amplitude modulation. And that pedestal is a problem in the original saturated spectroscopy and the intermodulated fluorescence spectroscopy. Meanwhile, there is no asymmetry of the line-shape in the polarization spectroscopy, because only the fluorescent or opto-galvanic signal are detected in this new method. In this paper a study of velocity-changing collisions in POLINEX spectroscopy using the rate equation approach is presented. A kernel function is introduced to describe the probability of atom moving out or in the state v (velocity) and m (magnetic quantum number) and the related terms additional to the rate equations. We calculate the total absorption rate and pick out the term concerning the mutual absorption (intermodulated signal). The amplitude of signal is proportional to the difference of the absorption rate when the senses of polarization of two counter-propagating laser beams are same or perpendicular to each other. We obtain the results which show the term related with the Doppler-broadened pedestal being cancelled after the operation of Clebsch-Gordon coefficients. These results coincide with the experiments performed before. Using the density matrix approach we can obtain the same results.