DOI: 10.2514/1.48867 Based on vortex–acoustic coupling theory, large-eddy simulation with wall-adapting local eddy-viscosity model and finite element method are carried out to study the internal flowfield and acoustic field, respectively, in a tailpipe nozzle solid rocket motor with transition-section grain configuration. The numerical method by means of a mesh sensitivityanalysisisproposedforvalidation.Theinstantaneous flowfieldcharacteristicsinthecombustionchamber and tailpipe are analyzed. The excited low frequencies are close to that observed in experiment. The phenomenon in which acoustic signals, superimposed on the vortex-shedding motions, couple with an internal flowfield is proven to be one of the main reasons contributing to oscillation in the motor. According to fast Fourier transform, low frequenciespredominateinthecombustionchamber;however,highfrequenciespredominateinthetailpipe.Dozens of cases with different geometrical configurations are presented to investigate parameters that have impact on the low-frequency oscillation characteristics. The results indicate that the oscillation characteristics are mainly influenced by upstream mean velocity, transition-section angle, distance between vortex source and impingement points, tailpipe radius, and convergence angle of the nozzle.