Polyoxymethylene (POM) is a good absorber of CO2 laser, so it is important to study the ablation mechanisms of polymer materials. Because the laser impact phenomena are terribly complex, there is no general understanding of the mechanism of laser induced ablation of POM. An explicit thermal-chemical coupling model is presented in this paper, which takes account of laser heating, phase transition, thermal degradation, and plume emission. Random thermal degradation is adopted to describe the chemical reaction process when POM is heated up, and consequently, the components of the degradation products under different degradation rates are acquired. The group contribution method is used to evaluate the thermodynamic properties of the degradation products, and the normal boiling point and critical temperature of the product mixture are obtained by the mixing law. If the product temperature is lower than the critical temperature, POM is ablated in the manner of liquid evaporation; otherwise the ablation mechanism is gas-dynamics emission. As for the former, Knudsen layer relationship is employed to calculate the ablation mass; and for the latter, the conservation laws associated with the Jouguet condition are used. Based on the model, the quantitative results of ablation mass, ablation temperature, product component and mass rate of different ablation mechanisms vs. laser fluence are achieved and analyzed, which are consistent with the experimental data quite well.