To study the horizontal bearing capacity of pile with cap under the existing vertical load, two groups of PIV model test were carried out. The effects of different vertical loads on the horizontal displacement, bending moment of pile and soil displacement field around pile are analyzed. The influential mechanism of vertical load on pile-soil interaction under pile with cap is studied. The results show that the existing vertical load decreases the horizontal displacement, weakens the bending moment of pile under the same horizontal load. In the process of horizontal loading, the main region of pile-soil action is the middle and upper region of the soil around the pile. The application of the vertical load on the pile top will mobilize the soil around the pile, form a larger soil disturbance area. The soil densification effect caused by vertical load will reduce the horizontal displacement and bending moment of pile and increase the horizontal bearing capacity of cap pile. Meanwhile, this densification phenomenon also reduces the uplift and collapse area of pile side soil.