Based on the Hoek-Brown strength criterion, a calculation model of pile lateral normal stress and horizontal friction under small rotation is obtained. Using static equilibrium principle, a calculation formula for the rock-socketed depth of the highway bridge pile foundation under horizontal load is established. Furthermore, a new calculation method of rock-socketed depth is proposed. The analyses of parameter sensitivities and influence factors show that: (1) the torque caused by horizontal load, pile diameter d, strata overlying pressure , rock uniaxial compressive strength , rock mass category parameter and the rock geomechanical classification index RMR have certain influences on the rock-socketed depth. While , d are unchanged, the rock quality and rock uniaxial compressive strength are the most sensitive factor to determine of rock-socketed depth. (2) The worse the rock mass quality is, the greater the rock-socketed depth is; the better the rock mass quality is, the smaller the influence of overlying pressure on the rock-socketed depth is, vice versa. (3) The rock-socketed depth nonlinearly decreases slowly with the increasing of rock uniaxial compressive strength, and under the same uniaxial compressive strength ,the better the rock mass quality is, the smaller the rock-socketed depth is, vice versa.