In order to study the bearing behavior of a single pile under combined vertical-torsional（N-T）loads in sand overlying clay soils, based on an indoor model test, a series of load-displacement curves and inner force distribution of pile shaft under combined N-T loads were obtained. By dimensionless treatment and curve fitting, the envelope of pile bearing capacity and its simplified calculation formula were obtained. Further numerical simulation on the single pile under combined N-T loads was carried out by ABAQUS to find out the influence law of elastic modulus（Ep），slenderness ratio（L/D）and relative thickness of the sand layer（lu/L）on the pile behavior. Results show that compared with a pile under a single load, the ultimate vertical and torsional bearing capacity of pile under combined N-T loads obviously decrease, and applying N（or T）causes the increase in twist angle（or settlement）of the pile shaft. It is also found by numerical analysis that pile deformation mainly occurs in the range of 0~0.6L，increasing L/D and lu/L causes the bearing capacity of the pile shaft significantly increase, but increasing Ep has little effect on improving the pile capacity. Therefore, it is suggested that the effective method to improve the bearing capacity of the pile shaft is to appropriately increase pile length or thickness of the sand layer. Additionally, obvious sudden changes of stress occur at the interface between upper and lower soil layers.