As the source rock and reservoir of coalbed methane (CBM), coal bed is different from conventional gas reservoirs in its dualporosity rock composed of matrix pore and fracture and being of its unique cleat system. The size, shape, porosity and connectivity of matrix pore and cleat determine accumulation, migration and production of CBM, with cleat system being most important for CBM production. Taken the Qinshui coal field as an example, this paper focuses on the type of coal cleats and fillings, fillings way, and formation age of authigenic minerals of coal cleat. Based on formation age of filling and burial history of coal bed, three formation mechanisms of cleat were proposed: buried pressurization mechanism; magma induced mechanism; uplift depressurized mechanism.