In this study, the influence of biomass ash (rice straw ash, RSA) additive on char gasification reactivity of different rank coals (Shenfu bituminous coal and Zunyi anthracite) was investigated using thermogravimetric analysis. Moreover, the structure characteristics (i.e., the chemical forms and concentrations of AAEM species and the order degree of carbon structure) of gasified semichars were quantitatively studied to evaluate the effect of RSA additive on coal char structure evolution during gasification. Specific reactivity index was proposed as a quantitative index and showed that RSA additive facilitated coal char gasification, especially at lower gasification temperature and for high-rank coal char. In addition, the results from the active AAEM concentrations and the Raman band area ratios of gasified semichars indicate that the RSA additive was conducive to the increase of active AAEM concentrations in coal char and the decrease of the degree of graphitization of coal char carbon structure during gasification, which were more significant at lower temperature and for high-rank coal char. It could be concluded from these results that the function mechanism of the RSA additive on coal char gasification reactivity had a close relationship with char structure evolution during gasification. Kinetics analysis using isoconversional method demonstrated that the gasification activation energy of Shenfu and Zunyi coal char with RSA additive were respectively lower than those of the corresponding coal chars by 8.33 and 22.32 kJ mol-1, indicating that the RSA additive was favorable for activation energy reduction of coal char gasification, especially for high-rank coal char. This work verified the possibility of promoting coal gasification using biomass ash as a natural catalyst and revealed the function mechanism of biomass ash additive on coal char gasification.