*† ‡ § ** In this paper, simple reacting flow model of unchoked solid-propellant ducted rocket combustor based on Eddy Break-Up model is established and used to simulate an experimental motor. The three-dimensional Favre-averaged compressible turbulent N-S equations were used as the governing equations, the improved e ~ k two-equation turbulence model was used to simulate the turbulent flow, and Eddy Break-Up combustion model was used to simulate the gas combustion. The mixing processes of air with fuel-gas in the secondary combustor were numerically studied with flux-vector splitting scheme. The secondary combustion in a 120mm-diameter cylindrical combustor for the solid unchoked ducted rocket was investigated preliminarily conducted on a direct-connect test bed. During the test, aluminum/magnesium-loaded fuel-rich HTPB propellant was used. The effect of factors, such as air/fuel ratio, velocity of air injecting, dome height, angle of gas injector and others on the performance of the combustor and the engine was examined. The work described in this paper represents an attempt to direct the design of second combustor in order to increase combustion efficiency.