To address the challenges of ejection seat testing under high-speed conditions, full-scale rocket sled testing serves as a low-risk and cost-effective research method. To determine the impact of cavity structures on ejection escape rocket sleds, this study establishes a simulated forward fuselage incorporating a cavity structure. Based on the SST turbulence model, numerical simulations were conducted to analyze the aerodynamic characteristics of the cavity structure in the simulated forward fuselage of the ejection escape rocket sled. The results indicate that under subsonic, transonic, and supersonic conditions, two vertically aligned large vortices form inside the cavity. A portion of the shear layer collides with the rear wall of the cavity, generating impact shock waves, and the oscillation amplitude in the rear region of the cavity is significantly higher than that at the front edge. The cavity structure induces drag oscillations of approximately 10% across different Mach numbers, while the aerodynamic drag increases by over 20% compared to rocket sleds without cavity structures.