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The single-expansion ramp nozzle (SERN) has a compact structure and adaptability under off-design conditions, which is one of the nozzle configurations that the combined cycle engine gives priority to. Flow separation is likely to occur when the nozzle operates at low-pressure ratios. Subsonic regions, such as separation bubbles, recirculation zone, and boundary layer, also exist in the overexpanded flow field. At this point, the varying external flow field can affect the flow field inside the nozzle, which may lead to the transition of the flow separation pattern. The majority of relevant studies are conducted in a static environment because of experimental conditions limitations. This paper numerically simulates the transitions of separation patterns during the acceleration and deceleration processes of a short flap SERN. The separation transitions mechanism is analyzed, as well as the performance impact of the transitions. Current results show that the separation pattern transition happens during both the acceleration and deceleration processes, and free shock separation is a relatively stable state. The separation pattern transition occurs as a function of the external flow field, recirculation zone, boundary layer, and separation shock wave coupling. Compared with previous studies on the long flap SERN, the coupling mechanisms of the internal and external flow fields of the two nozzles were found to be different. The shock wave structure of the short flap SERN's flow field is stable during the transition processes, and the nozzle performance does not exhibit a step change. The hysteresis of the short flap SERN is also weaker. [ABSTRACT FROM AUTHOR]