A mixture of Pingdingshan lean coal and acidtreated Huadian oil shale was co-pyrolyzed in a droptube fixed-bed reactor in the temperature range of 300 ℃-450 ℃.To reveal the formation mechanism of the solid co-pyrolysis product,changes in some physicochemical properties were investigated,using analysis by X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy,pore analysis,thermogravimetry,and electron spin resonance.X-ray diffraction showed that the lattice plane spacing for the co-pyrolyzed mixture decreased from 0.357 nm to 0.346 nm and the average stacking height increased from 1.509 nm to 1.980nm in the temperature range of 300 ℃-450 ℃,suggesting that pyrolysis treatment increased its degree of metamorphism.The amount of oxygen-containing functional groups and pore volume decreased with increasing temperature.Thermogravimetry and electron spin resonance results showed that synergistic effects occurred during the copyrolysis process.A formation mechanism for the solid product was proposed.Hydrogen-rich radicals generated from the pyrolysis of the oil shale were trapped by hydrogen-poor macromolecular radicals of the intermediate metaplast produced from coal pyrolysis,thereby increasing the yield of solid product.