In this paper, the endwall film cooling and vane pressure side surface phantom cooling performances of a first nozzle guide vane (NGV) with endwall contouring, similar to an industry gas turbine, were experimentally and numerically evaluated 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). A novel numerical method for the predictions of adiabatic wall film cooling effectiveness was proposed based on a double coolant temperature model. The credibility and accuracy of this numerical method were validated by comparing the predicted results with experimental data. Results indicate that the present numerical method can accurately predict endwall film cooling performance and vane surface phantom cooling performance for both the ideal low density ratio (DR = 1.2) and the typical high density ratio (DR = 2.0) conditions. The endwall film cooling effectiveness, vane surface phantom cooling effectiveness, and secondary flow field were compared and analyzed for the axisymmetric convergent contoured endwall at three coolant injection angles (small injection angle of θ = 40 deg, design injection angle of θ = 50 deg, large injection angle of θ = 60 deg), two density ratios (low density ratio of DR = 1.2 and typical high density ratio of DR = 2.0), and the design blowing ratio (M = 2.5), based on the commercial CFD solver ansys fluent. An analysis method of the coolant momentum flux φ (decomposed into the axial component φx and vertical component φz) was proposed to describe and explain the migration and mixing mechanisms of coolant flow. Results indicate that the proposed analysis method of the coolant momentum flux φ can accurately and reliably describe and explain the coolant flow physics, including the coolant lift-off, the interaction between the coolant and the mainstream, and the coolant migration in the vane passage. The increased coolant injection angles result in a deterioration of the endwall film cooling performance and vane pressure surface side phantom cooling performance. Nevertheless, the sensitivity of endwall film cooling effectiveness and phantom cooling effectiveness to coolant injection angles is variable, and is significantly affected by density ratio. This suggests that the coupling effects of the coolant injection angles and density ratio should be taken into account for the prediction of endwall film cooling and phantom cooling performances. It is very necessary for the optimized design of the coolant injection angles and the predictions of film cooling performance at the realistic density ratio.