The particle diameter had an important influence on the rate of gas-solid reaction. The chemical reaction mechanism of K/CO3 sorbents absorbing CO2 was analyzed in a reconstructive thermogravimetric analyzer and the effect of the diameter on the sorbents conversion rate was investigated. A shrinking core model was developed to describe the reaction of CO2 with K2CO3 particles. The inner diffusion coefficient and reaction rate were calculated through thermogravimetric data, which were key parameters in the model. Meanwhile, an association relationship between diffusion coefficient and conversion rate was created. The result shows that the transformation from reaction control step to inner diffusion control step is observed in sorbents with all sizes. The increase of diameter is harmful to the reaction and the diffusion coefficient decreases with the increase of the conversion rate. The accordance of model data and experiment data verified the accuracy of model which provided a basis for a further study.