Abstract: A limited number of exploration wells have been drilled into Baoshi Formation in Xihu Sag, East China Sea, and the relevant geological data obtained is so limited that there has been few studies on Baoshi Formation. According to previous studies, the lithology of Baoshi Formation is mainly composed of mudstone, sandstone and tuff. Well B7 deployed in Area B was drilled through Baoshi Formation into the basement and cored in Baoshi Formation. Based on the data of cuttings, cores, logging data, etc., the following new understandings on the lithology of Baoshi Formation in this area have been made: (1) The local lithology at the top of Baoshi Formation develops kaolinite clay rock, and the cuttings are similar to tuff, which can be distinguished by microscope, and the logging characteristics are also different. (2) Granite intrusion occurred in some strata near the basement in the lower part of Baoshi Formation, and the mudstone was subjected to thermal contact metamorphism to form the hornstone. At present, the new lithology only appears in the well B7, and the study on its distribution range still needs more drilling to provide physical data in the later stage. The drilling of Well B7 has obtained a new understanding of the lithology of Baoshi Formation, and provided new data for subsequent drilling, mud logging, and geological research.