Second throat supersonic exhaust diffusers (STEDs) were designed to simulate high-altitude conditions according to the normal-shock model. Experimental studies were performed on the STEDs to investigate how performance characteristics varied with the length and diameter of the STED using high-pressure nitrogen gas. The variation in performance due to length indicated that the performance of the STED could be very slightly improved by adjusting the diffuser inlet length ( ), and it could be significantly improved by optimizing the second throat length ratio (   ) and the divergence length ( ). The starting and vacuum chamber pressures exhibited the highest level of performance near     of the design point.