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A thermogravimetric analyzer and in-situheating microscope were used to study gasification reaction of coal char particles on the interface of ash layer and slag at 1100, 1200 and 1300°C, using typical Ningdong coal-Yangchangwan coal as gasification raw material. The results show that the shape change of ash layer interface and slag interface under different gasification temperature is the main factor that affects the gasification reactivity of coal char particles. When the gasification temperature is 1100°C, the ash layer shrinks and wraps on the surface of coal char particles at high temperature, preventing the contact between gasifying agent and coal char particles and reducing the gasification reaction of char particle. However, the interface of slag does not change significantly, and the gasification reaction rate of coal char on the slag interface remains unchanged. When the gasification temperature is 1300°C, the interface of both ash layer and slag turns into a liquid phase. Under the surface tension, the char particles are broken, the effective reaction area became larger, and the heat transfer rate increases, thereby increasing gasification reaction rate of the coal char.