An expansion tube/tunnel is a ground-based test facility to generate hypervelocity test flow for the study of reentry physics. A detonation-driven expansion tunnel (JF-16) has been built at the State Key Laboratory of High-temperature Gas Dynamics (LHD), in order to generate relatively steady and clean test flows at high enthalpies. However, the extremely short test time leads to critical difficulties inflow analysis and measurement. Numerical simulation is a powerful assistant analysis tool for the study of hypervelocity test flow and its diagnostics. With the development of hypervelocity optical diagnostic technique, a flow visualization system has been set up to capture the flow field structure. The establishment and development processes of the flow field for expansion tunnel can be analyzed directly through the images obtained by the high-speed camera. At the same time, the standoff distance of the detached shock wave and angle of the oblique shock wave have been measured to validate the numerical techniques.