The morphological evolution of a single char particle with a low ash fusion temperature (T-f) was investigated during the whole gasification process using a high-temperature stage microscope. The experimental results showed that the final shrinkage ratio of char particles at the temperature above Tf (1300 degrees C) was higher than that below the deformation temperature (1000 degrees C). The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analysis results showed that the ash distribution in coal char was uniformly dispersed. Dispersed molten slag gradually aggregated to the molten slag layer during the gasification process at the temperature above T-f, and the slag layer evidently hindered the diffusion of the gasifying agent. Therefore, there was a critical shrinkage ratio during the gasification process when the temperature was above T-f and the critical shrinkage ratio of Xiaolongtan lignite (XLT) char particles was 0.7-0.8. The SEM energy-dispersive X-ray spectroscopy (EDS) analysis results indicated that the critical points in the curve of the shrinkage ratio with time were related to the molten slag on the char surface. In addition, the reaction mechanism in this experiment was analyzed. The calculating results indicated that the critical thickness of the molten slag layer of XLT and Shenfu bituminous (SF) coal chars with different ash contents showed good agreement and the critical thickness of the slag layer was 6-18 mu m. Finally, the sensitive analysis was performed to analyze the factors affecting the calculating results.