Introduction  The aim of this paper is to analyze the influence of the size and the vertical load of the pile foundation on the horizontal bearing capacity of rock-socketed piles.

  Method  Based on a field test of the rock-socketed concrete pile under horizontal load， a 3-D finite difference numerical model was established and parametric analyses were performed.

  Result  The results show that there is a critical rock-socketed length （ratio of length and diameter） for rock-socketed piles. Changes of the size of the pile will influence the relative stiffness of the pile and the rock， which will affect the critical rock-socketed length. With the decrease of the pile diameter， the critical socket length will increase. Within the scope of the bearing capacity of the pile and under the condition of combined vertical and horizontal load， the horizontal displacement and bending moment of the pile will have a pretty large decrease. This influence will be neglected if the vertical friction is assumed to be the same on the both sides of the pile or simplified to be at the center of the pile， which will cause error to the calculation results.

  Conclusion  This study is supposed to provide contribution to the reasonable design and further research of the horizontal load of rock-socketed piles.