In 2021, Hebei Geological Survey Institute discovered an extraordinary enrichment of lithium in the Middle Mesoproterozoic Wumishan Formation in the eastern Hebei, which is the first time that clay type lithium resources were found in Precambrian strata in China. The lithium-rich rock series has large thickness, wide distribution, stable horizon, Li2O grade of 0.1-0.42 wt%, and has great prospecting potential. As a new lithium-endowed horizon, the occurrence state of lithium is directly related to the exploration and development significance of the clay-type lithium resources discovered this time. By using X- ray diffraction (XRD), thermal analysis (TG-DTG) and Fourier transform infrared spectroscopy (FTIR), it is found that the lithium-rich rocks in the study area are mainly composed of dolomite, quartz and clay minerals such as illite and illite/smectite mixed layers. Through solid state nuclear magnetic resonance spectroscopy, time-of-flight secondary ion mass spectrometry (TOF-SIMS), clay mineral purification and Na ion exchange experiments, it is revealed that the lithium in the Wushan Formation lithium-rich rock series mainly exists in the illite and illite/smectite mixed layers in the form of isomorphic replacement, and a very small amount of lithium exists in the form of ion adsorption between the clay mineral layers or on the surface. The research results provide important theoretical basis and technical support for the selection, metallurgy and exploitation of clay-type lithium resources in eastern Hebei Province in the future.