The deep coalbed reservoirs in Yichuan area exhibit developed cleats and strong rock heterogeneity, leading to frequent occurrences of wellbore instability during drilling, particularly in the build and horizontal sections. Analyzing the main controlling factors and control methods for reservoir instability is crucial for successful drilling operations in deep coalbed gas reservoirs. Therefore, this study experimentally analyzed the rock mechanics properties and fundamental physical characteristics of different strata from the roof to the bottom of the No. 8 coal seam in the Yichuan area. The study investigated the influence mechanism of drilling fluid hydration on the rock mechanics properties and micro-pore structure of coal seam rocks. Based on the characteristics of the No. 8 coal seam, we optimized the design of fluid loss agents, plugging agents, and inhibitors in drilling fluids and developed a drilling fluid system suitable for controlling wellbore stability in deep coal and rock gas reservoirs. The research results indicate that the roof rocks are dense and hard, with compressive strength significantly greater than that of the coal seam; the Poisson's ratio of the coal seam is large and the elastic modulus is much smaller than that of the roof and bottom coal, indicating strong rock plasticity. The coal and rock have a low water absorption capacity, but the water absorption by kaolinite leads to significant rock expansion, with a volume expansion rate exceeding 5%. The immersion of drilling fluid has a minor impact on the roof but results in hydration expansion of coal seam flaky kaolinite aggregates, widening micro-cracks. Under thermal stress, coal integrity is compromised, micro-cracks increase in number, and overall rock strength decreases. The long-term immersion of rocks must be considered in the post-horizontal drilling stage to assess its impact on the safe density window of the drilling fluid.