The kinetic models based on complex free-radical mechanisms always involve lots of parameters, which result in model overparameterization. In this work, on the basis of free-radical reaction mechanisms, a simplified kinetics for liquid-phase catalytic oxidation of p-xylene (PX) to terephthalic acid (TPA) was developed. By assuming that different peroxy radicals have equivalent reactivity, all the initiation rate constants are identical, and the differences in the rates of termination between various peroxy radicals are neglected, the kinetic model is simplified to include only six parameters that are to be determined by experiment. The kinetic model established in this paper was shown to have satisfactory precision in predicting the concentration profiles. The kinetic model proposed is even simpler than the first-order kinetic model because the rate constants concerning chain propagation and termination are independent of temperature within the range investigated.