Monopiles are the most common foundation form of offshore wind turbines, which bear the vertical load, lateral load and bending moment. It remains uncertain whether the applied vertical load increases the lateral deflection of the pile. This paper investigated the influence of vertical load on the behaviour of monopiles installed in the sand under combined load using three-dimensional numerical methods. The commercial software PLAXIS was used for simulations in this paper. Monopiles were modelled as a structure incorporating linear elastic material behaviour and soil was modelled using the Hardening-Soil (HS) constitutive model. The monopiles under vertical load, lateral load and combined vertical and lateral loads were respectively studied taking into account the sequence of load application and pile slenderness ratio (L/D; L and D are the length and diameter of the pile). Results suggest that the sequence of load application plays a major role in how vertical load affects the deflection behaviour of the pile. Specifically, when L/D ratios obtained by lengthening the pile while keeping its diameter constant are 3, 5 and 8, the relationships between lateral load and the deflection behaviour of the pile under the effect of vertical load demonstrate a similar trend. Furthermore, the cause of increased lateral capacity of the pile under the action of applied vertical load in the common practical application case and in the VPL case was analyzed by studying the variation law of soil stress along the pile embedment. Results confirm that the confining effect of vertical load increases means effective stress of the soil around the pile, thus increasing soil stiffness and pile capacity.