In accordance with the flow characteristics of vortex valve variable-thrust solid rocket motors, a cold flow experimental system based on Particle Image Velocimetry was established. A flow velocity vector diagram of vortex chamber was generated, and the vortex structure was analyzed. The results provided an experimental foundation for numerical simulation. The flow characteristics in vortex chamber and in the throat and divergent sections of the nozzle were modeled and simulated. The flow in the vortex chamber conformed to the complex Rankine vortex, and the flow field was divided into three different zones. The vortex core was the primary influence factor for thrust modulation. The resultant velocity reached Mach number 1 before gas arrived at nozzle throat, and the axial velocity still reached Mach number 1 at nozzle throat. Hence, the axial velocity can be used to judge the occurrence of choking at the nozzle throat. The intensity of swirl flow in divergent section of the nozzle was evidently lower than that in vortex chamber and throat. As a result, a low-pressure zone emerged around the central axis, thereby causing thrust losses.