It is generally acknowledged that electromagnetic radiation (EMR) phenomenon occurs during coal or rock loading and failure. This physical phenomenon can be used as a method for monitoring and early warning of dynamic hazards underground spaces. Though many hypotheses have been proposed to explain EMR phenomenon, the mechanism of EMR is still far from clarity. In fact, the occurrence of EMR is closely related to the surface microstructures of coal or rock. Thus, it is necessary to explore the surface electrical property of coal from the micro level. In this paper, 10 coal samples with different rank were selected for experiments. Fourier transform infrared (FTIR) spectroscopy was used to determine the surface microstructures of coal, and the surface potential was measured by atomic force microscopy (AFM). Subsequently, the effect of coal microstructures on surface electrical property was analyzed, and the correlations between these two parameters were also established. The research results showed that coal rank has significant influence on microstructures. Both the content of C=C groups and C=O groups is increased with the enhancement of coal rank. However, both A(C-O) and A(OH) decline rapidly with the increase of Ro. The heterogeneous distribution including both positive and negative potential zones of coal surface potential at nanoscale level was discovered through AFM measurements, and the net surface potential (NSP) is negative overall. It is also found that NSP is 2-3 magnitudes less than that of nanoscale level. This suggests that coal surface has relatively obvious electrical property at nanoscale, but shows weak electrical property and even electric neutrality at the mesoscopic and macroscopic levels. This phenomenon is called size effect of coal surface potential. Besides, the effect of microstructures on coal surface electrical property was analyzed, finding that NSP is positively correlated with the content of both C=C groups and C=O groups, but C-O and OH groups can reduce the value of NSP. The electronegativity of microstructure presents a strong correlation with the total negative surface potential. The stronger polarity of groups corresponds to the greater electronegativity. Electron gain or loss is more readily to occur for these groups with higher polarity under loading and failure, as a result, coal or rock EMR is generated due to charge transfer. This study enables us to better understand the EMR mechanism at the microscopic scale, as well as provides guidance for the monitoring and early warning of underground mining safety.