A considerable amount of expensive polyamide 12 material remains unjoined in selective laser sintering (SLS) additive manufacturing. Such materials, particularly the ones near the heat-affected zones, go through irreversible chemical degradations originated from thermal oxidations. In the SLS of polyamide 12, despite efforts in understanding the degradation mechanisms of the materials, full modeling of the complex material degradation remains not well understood. In this work, a first-instance kinetic model is proposed considering the effects of both oxygen and lasers to model the material degradation in SLS. By a combined theoretical and experimental mapping of the actual material degradation rates into the oxidation physics and parameter identification, the coefficients of the actual coupled oxygen and laser effects in SLS have been obtained. Through sensitivity analysis, the fitting equations between the sample degradation rates and the oxidation time have been derived. The proposed kinetic model can predict the oxidation rates of pure or mixed materials using two easily available parameters: materials density and oxidation time. Moreover, the results suggest that laser effects are four times stronger than oxygen effects on polyamide 12 degradation. The predicted oxidation matches, on average, 89.53% with the actual SLS degradation rates, in contrast to a 34.48% accuracy from a basic autoxidation model.