The cogasification reactivity of Shenmu bituminous coal (SM) char and corn straw (CS) char in CO2 was evaluated by a fixed-bed/fluidized-bed reactor and mass spectrometry. The effects of blend ratio, gasification temperature, and CS char volatiles on the cogasification reactivity were characterized by a synergistic index The results show that the interaction of the cogasification char with low temperature and low CS blend ratio changed from inhibition to promotion with the increasing conversion. The increase of the CS blend ratio and gasification temperature can weaken the inhibition and enhance the synergy. Furthermore, CS volatile-potassium (K) could promote the cogasification reaction, and the promotion effect increases with the rise of temperature. Finally, the cogasification reactivities of CS-SM blend char and CS char volatiles-SM char were compared. The results demonstrate that the reaction form of CS char volatiles-SM char has a better effect on improving gasification reactivity when alkali and alkaline earth metals are not melted. Otherwise, the reaction form of the CS-SM blend char performs better.