Modern gas turbine combustor outflow causes non-uniformities to the high pressure turbine, including hot streak and swirl, which will directly influence the flow structure and cooling performance of the nozzle guide vane (NGV) endwall. In this paper, the cooling performance of double row cooling holes at the leading edge area of the endwall is numerically investigated with consideration of a combustor simulator exit conditions. The effect of coolant mass flow ratio (0, 0.5%, 1.0%, 1.5%, 2.0%, 3.0% and 5.0%) of the double row holes on the heat transfer and cooling effectiveness is studied. Furthermore, circumferential positions of the swirl center to the NGV leading edge are compared, including passage-oriented and leading-edge-oriented clockings. It is found that the difference of heat transfer and cooling characteristics under different clocking positions is less obvious on the hub. As for shroud, the clocking effects mainly change the lateral redistributions of the cooling performance. The endwall cooling performance is mainly affected by changes of coolant MFR. The cooling effectiveness improves a lot when the coolant momentum increases to be able to drive the coolant pass through the separation line of the pressure side horseshoe vortex.