This work examined the reaction behavior and kinetics of in situ char gasification and the corresponding biomass devolatilization by TGA based on the non-isothermal method. Four kinds of devolatilization atmosphere (N-2, N-2+CO2 (10%), N-2+CO2 (50%), N-2+steam (10%)), three kinds of gasification atmosphere ( N-2+CO2 (10%), N-2+CO2 (50%), N-2+steam (10%)), and different heating rates (5, 10, 20, 30, 40 K/min) were adopted. With the increase of heating rate in each atmosphere for biomass devolatilization or char gasification, the typical reaction temperatures, including TBD-max, TCG-i, and TCG-max , increased obviously due to the delayed effect. At a given heating rate, the adopted devolatilization atmospheres displayed very weak influence on the volatiles release behavior and kinetics, even for the CO2 content of 50%, but had an obvious influence on the char property. The volume reaction model and shrinking core reaction model were suitable to describe the behaviors of biomass devolatilization and char gasification, respectively. Compared to biomass devolatilization, the reaction atmosphere had a remarkable effect on char gasification, especially in the curve shape of reaction rate and kinetic parameters. For the in situ char gasification, the calculated activation energy (E) in the steam atmosphere was much lower than that in the CO2 atmosphere. While for the char gasification in the steam atmosphere, the E of ex situ char was higher than that of in situ char, indicating the necessity of adopting the in situ char for the char gasification research.