As one of the widely utilized renewable energy of environmental protection materials, straw can generate electricity by burning directly. Mathematical modeling methods have been employed to simulate a straw-fired grate boiler, not only the temperature, velocity and concentration distributing of the top of bed in different conditions, but also the burning rate of bed and burning character of furnace chamber were calculated, by varying the ratio of primary air and the second air. The results showed that when the ratio of primary air and the second air is 1.3∶0.7, the lowest value of concentration of CO is 0.29 mol/m3, the average temperature of furnace chamber outlet is 1 180 K, and concentration of O2 is 2.18 mol/m3. It is proved that the simulation results compared reasonably well with the relative literature measurement data. 