The present study aims to investigate the flow and cooling characteristics in second throat ejector diffuser used in the High-Altitude Testing (HAT) Facility using gaseous film cooling technique. A detailed computational analysis with three different coolant injection configurations (normal, inclined, and tangential) and with varying mass flow rate ratios (ratio of coolant to ejector-diffuser mass flow rate) were performed. It was found that the injection of secondary fluid into the mainstream produced additional secondary shock structure apart from the primary shock structure produced by the supersonic nozzle flow impingement with the diffuser wall. The secondary shock wave reflection was clearly observed for the normal and inclined injection configurations, whereas for the tangential injection configuration the secondary shock structure was not observed. It was found that the cooling effectiveness of the normal injection was higher than the inclined and the tangential injection for the same mass flow rate ratio. The film cooling effectiveness increases with an increase in mass flow rate ratio for all the injection configurations. It was found that, with the highest injection mass flow rate considered in the present study, the average diffuser wall temperature drops by nearly 40% for all the injection cases. It was also observed that the coverage length is higher for normal injection followed by inclined and the tangential injection. The coverage length has been considered to be the distance at which the diffuser wall attains 900 K (60% temperature of that of the without injection case (1500 K)).