The Jianan region is located on the eastern edge of the Ordos Basin, where there are developed geological fractures, low bearing capacity, and high clay content in coal seams. Complex drilling situations often occur in deep coal seam drilling. To ensure the safe drilling of horizontal sections in deep coal seams, rock characteristic tests were conducted. By comparing and analyzing the changes in the microstructure and mechanical parameters of rocks before and after immersion in drilling fluid, the variation rules of mechanical characteristics of deep coal series formations under the influence of drilling fluid immersion in the western exploration area were summarized. The research results indicate that the clay mineral content of carbonaceous mudstone roof and interlayer clay minerals is between 13 and 16%, with a high proportion of illite and kaolinite, requiring precautions against hard brittle block falling; the clay mineral content of coal seams is between 5 and 15%, with a high proportion of kaolinite and illite-montmorillonite mixed layer, requiring precautions against clay hydration expansion. The size of most of the geological fractures in the formation ranges from 25 to 400 μm. After immersion in drilling fluid, the porosity of block rocks increased to varying degrees, ranging from 19.94 to 366.64%, and the range of pore radius expanded. The rock mechanical parameters all decreased to varying degrees. Clay minerals in the formation expand upon encountering water, leading to easier expansion and extension of pore fractures, causing instability of the wellbore, necessitating measures to maintain wellbore stability. The variation rules of deep coal rock mechanical characteristic parameters under drilling fluid immersion obtained in this study are expected to provide theoretical support for solving the problems encountered in the drilling of long horizontal sections in deep coal seams in the Jianan block.