According to field investigation of mercury slags in Tongren area, this paper revealed that dump slags, tailings and cementing substances are three major constituents of cemented mercury slags and analyzed the material compositions of each constituent. Based on this, by applying the principal component analysis and field cementing test, it identified the influence of each factor on the sensitivity of the development of cementing substances. In descending order of importance, these are hydrological condition, accumulation years, ventilation, overburden thickness, position types, light and porosity. Later on, cementing types of the mercury slags were grouped, which include A type powder cementing of total gravels, B type powder cementing of local soft rocks and gravels, C type powder cementing of soft rocks and gravels, D type gravel cementing with powder residue, E type all-inclusive gravel cementing, F type contact-type anomalous gravel cementing and G type contact-type spherical gravel cementing. Finally, the contents of cementing substances with different material compositions were explored. A study on the cementing substances in mercury slags and their composition relation will provide significant theoretical and engineering value to the prevention of slag accumulation stability.