Abstract: Ximeng lignite and cornstalk were used as the feedstock to prepare lignite char, biomass char and co-pyrolysis char with different blending ratios in a fixed bed reactor with temperature-programmed pyrolysis. The pore and chemical structure of char samples were characterized and the ash composition was analyzed. The oxidation reactivity of the mixtures of lignite char/cornstalk char with different blending ratios and the co-pyrolysis char of lignite and cornstalk with corresponding blending ratios were investigated by the isothermal thermogravimetry at 450 ℃, aimed at the effect of co-pyrolysis process on the char reactivity. The results show that there are obvious influences on the char structures through secondary reactions during co-pyrolysis process, leading to the char reactivity decrease. Especially with the cornstalk proportion less than 50%, these influences are more significant due to a large number of volatiles from cornstalk during co-pyrolysis enhancing the secondary reactions between the volatile and nascent char, prompting parts of organic structure less than 5 rings turn into the larger organic structure. For the char samples with cornstalk proportion above 50%, the catalytic effect of alkaline and alkaline earth metal in biomass char plays a dominating role, especially the effect of potassium, resulting in the weaker effects of secondary reactions on the structure and oxidation reactivity of the char samples.