To analyze the influence law of existing vertical force（V） on the horizontal bearing capacity of piles in the medium sand and soft clay subsoil， a 3D finite element analysis model of a single pile was set up firstly by ABAQUS and then validated by using available loading test results. By keeping various constant values of V at the pile top and using the probe method， the horizontal load （H） was sequentially applied to obtain the lateral displacement， ultimate bearing capacities and bearing capacity envelopes of the shaft with two kinds of slenderness ratios （L/D） in homogeneous medium sand and soft clay， respectively. It was shown that， under the same variation range of V from 0.2 Vult to 0.8 Vult， the ultimate horizontal bearing capacity（Hult） of the pile in medium sand was increased by 13.6%~41.2%， while the value of Hult for soft clay subsoil was decreased by 2.2% to 6.5%. Furthermore， a more obvious effect was observed for the piles with a smaller L/D. Finally， the different influencing mechanisms of the existing vertical load on the horizontal bearing capacity of a single pile in the medium sand and soft clay subsoil were discussed in detail by analyzing the distribution and variation characteristics of the vertical stress in the subsoil， the side friction and lateral resistance along the pile shaft during the loading process.