To reduce the intensity of endwall secondary flow, the axisymmetric convergent contoured endwall is commonly designed in the first nozzle guide vane (NGV) passage of the real gas turbine engines. This endwall contouring can obviously alter flow field near endwall and affect the coolant flow through the upstream double-row discrete film holes. This leads to a significant influence on the endwall film cooling performance, vane surface phantom cooling, and vane passage aerodynamic performance. In this paper, a detailed numerical investigation on the endwall film cooling and vane pressure side surface phantom cooling was performed, at the simulated realistic gas turbine operating conditions (high inlet freestream turbulence level of 16%, exit Mach number of 0.85, and exit Reynolds number of 1.7 × 106). Based on a double coolant temperature model, a novel numerical method for the predictions of adiabatic wall film cooling effectiveness was proposed. This numerical method was validated by comparing the predicted results with experimental data of endwall Nusselt number, endwall film cooling effectiveness, and flow visualization near endwall. The results indicate that the present numerical method can accurately predict endwall thermal load distributions, endwall film cooling distributions, and vane surface phantom cooling distributions. The endwall heat transfer coefficient, endwall film cooling effectiveness, phantom cooling effectiveness of the vane pressure side surface, and total pressure loss coefficients (TPLC) were predicted and compared for two endwall contouring shapes (flat endwall and axisymmetric convergent contoured endwall) at three different blowing ratios (low blowing ratio of BR = 1.0, design blowing ratio of BR = 2.5, and high blowing ratio of BR = 3.5) with a constant density ratio of DR = 1.2, based on the present novel numerical method. Results show that the axisymmetric convergent endwall contouring leads to a slight enhancement (maximum enhancement level less than 20%) of endwall heat transfer in the entire vane passage (0 < x < 0.65Cx). The axisymmetric convergent endwall contouring has a significantly desired effect on endwall film cooling performance (maximum enhancement level of 67%), phantom cooling performance of the vane pressure side surface (maximum increase level approximately 100%), and aerodynamic loss (maximum reduction level of 1.45%) for all blowing ratio cases, but the benefit enhancement level is obviously affected by the blowing ratio values. This suggests that the optimum of endwall contouring shapes is an effective technical way to improve endwall film cooling performance and decrease the depletion of coolant; the coupled effects of the appropriate axisymmetric convergent endwall contouring and the optimum blowing ratio should be considered in the design process of advance endwall cooling schemes.