This study investigates the bearing capacities of group pile foundation (jacket foundation) installed on the seabed of offshore wind farm (OWF) at the Changhua coast of Western Taiwan for the jacket support structure of offshore wind turbine (OWT) using three-dimensional (3-D) finite element method (FEM). The jacket foundations are subjected to a combined Vertical-Horizontal-Moment (V-H-M) loading for the operational period. The responses of installed group pile foundations are investigated under the combined loading in marine silty sand-low plasticity silt & clay (SM-ML-CL) layers determined by 19 offshore boring logs. The validity of numerical procedures was verified by a large-scale lateral loading test of steel tubular model pile in laboratory. A systematic parametric study was performed to investigate the effects of the pile length L, pile diameter D, and pile spacing S on the ultimate bearing (or load) capacity behavior of the foundation. The effect of pile length is significant on the vertical bearing capacity (Vult) whereas pile diameter and pile spacing on the ultimate horizontal and moment loads bearing capacities (Hult and Mult). The normalized V-H and V-H-M failure envelopes of bearing capacity for the jacket foundations subjected to combined loadings can be expressed as functions of L, D, and S and fitted by elliptical shape curves. The V-H-M failure envelopes and approximated expressions are proposed to evaluate the mechanical stability of the group pile foundations for the jacket support structure of OWT under the combined loading condition.