The effect of oxygen on the thermal degradation of rice straw was investigated with a thermal gravimetric analysis (TGA) reaction system. The TGA curves indicated two sharp mass changes during the process and the reaction could be simplified as occurring in two stages. Oxygen speeds up the thermal degradation of intermediates. A kinetic model was proposed and its corresponding parameters such as activation energies, pre-exponential factors, and reaction orders were determined through experiments conducted in nitrogen gas (N2), 10% oxygen gas (O2), and air at the heating rates of 2, 5, and 10 K/min. The experimental data agreed well with those predicted by the proposed model, which validated the applicability of this model to the design of agricultural waste processing systems.