Abstract: Sticking often occurs in horizontal directional drilling (HDD), which not only affects the construction progress, but also results in economic loss. Analysis reveals that mud is the principal factor resulting in sticking. In respect to the carrying capacity of mud, this paper discusses five principal factors thereof ascertains its selection range. By means of controlling annular return velocity, improving drill speed, regulating appropriate mud density, improving mud rheological behavior and viscosity, the carrying capacity of mud for directional drilling of rock formation was improved, and the sticking probability was reduced. This provides precious experiences for the design and operation of similar engineering in the future. Based on laboratory experiments, mud formula suitable for directional drilling of rock formation was proposed. It was successfully applied in production, with favorable economic and technical benefits.