Zunyi anthracite was used as the raw material of gasification and rice straw ash and cotton stalk ash were optioned as biomass ash additives. Char-CO2 isothermal gasification experiments were conducted using TGA to investigate the effect of biomass ash addition on coal char gasification characteristics. Furthermore, the relationship between char structure evolution during gasification and char gasification reactivity was investigated. The results indicate that the biomass ash addition has a promotion effect on coal char gasification, which is well related to the increase in active AAEM content and the decrease in the order degree of carbon structure of chars by the addition of biomass ash. However, the promotion effect of biomass ash additives on coal char gasification is weakened with increasing gasification temperature, mainly due to that the positive effect of biomass ash additive on the increase of active AAEM content in char and the inhibition effect of biomass ash additive on carbon structure order degree of coal char become weaker at higher gasification temperature. Additionally, it is found that cotton stalk ash has a more significant positive effect on coal char gasification than rice straw ash because cotton stalk ash has a more obvious effect on the increase in active AAEM content and the decrease in the carbon structure graphitization degree of coal char.