The isothermal gasification of eleven raw biomass chars and six acid-washed biomass chars was performed under CO2 atmosphere, using a thermo-gravimetric analyzer (TGA) over the temperature ranges of 750~1000℃ and 800~1000℃, respectively. The dimensionless procedures of rc by using rc,0.2 were made. In 750~900℃ for raw chars and 800~1000℃ for acid-washed chars, the curves of dimensionless gasification reaction rate with carbon conversion rate for every char are overlapped at different temperatures after the carbon conversion rate reaches 0.2, which shows the gasification rates may be controlled by the similar evolution rules. Based on the simple collision theory (SCT) of chemical reaction kinetics, the kinetic rate expression of biomass chars during gasification reaction process was derived and activation energy of seventeen biomass chars was calculated. Combined with the catalysis theory, the empirical prediction model of activation energy for biomass chars was deduced on the basis of SCT. The results show that there is a good logarithmic relationship between the activation energy and the proportion of active sites occupied by the catalysts. It can be predicted that the intrinsic activation energy of the biomass char may tend to a certain value 254.35 kJ/mol if the catalytic effect of the metallic element is ignored, and the activation energy of the biomass char is about 66.02 kJ/mol for fully catalytic reaction.