The inner flow field of scramjet combustion chamber with fuel jet and cavity structure was simulated numerically on the basis of the 3 D Navier Stokes equations. Finite differential method was used. With a comparison between the numerical and experimental results, the unsteadiness of fuel jet and influence of cavity structure on the flow field were discussed. From the results, it is observed that the stable flow field of numerical simulation is similar to experimentation, and the unsteady dynamic phenomenon that fuel jet moves downstream and diffuses into the cavity is captured from simulation.