The vertical depth of Weirong deep shale gas reaches 3 800 m, and the formation collapse pressure coefficient is above 1.95. The ultra-long horizontal wells in the region are increasing year by year, and the ultra-long horizontal wells have become an important development means for stable production in the later stage of the gas field. Based on the research of rotary steerable drilling method, the hydraulic open-hole evaluation model and Landmark software are used to comprehensively analyze the influencing factors of geological parameters, trajectory profile, drill string mechanics, hydraulic parameters and surface equipment. The extension limits of the ultra-long horizontal section were different under different rig conditions and working conditions. The optimized profile type is beneficial to prevent collision between wells and safe running of pipe string. The stress intensity of the drill string in 3D trajectory profile is twice that of 2D trajectory profile, and both the density and viscosity of drilling fluid will affect the extension of ultra-long horizontal section. The evaluation results show that the theoretical limit of open hole horizontal section is 5 840 m without considering the drilling capacity, while the maximum horizontal section extension of 70 drilling rig is 3 500 m due to the drilling depth limit. The larger the pump displacement, the lower the drilling fluid density, and the longer the elongation capacity.