Theoretical and experimental studies on an imaging-based correlation velocimetry were presented.The signals received by detectors when a single particle passing through the measurement volume were calculated.The effects of particle size and detector diameter on the signal and its frequency spectrum were studied and the effective length of the measurement volume was analyzed.In data processing，the determination of the length of time window was studied and the methods using windowing functions and data pre-shift were also analyzed.Velocity measurements for the dense region of a spray near the nozzle exit by using the measurement system were successfully carried out.The spray velocity variations under different pulse injections as well as the radial and axial profiles of spray velocities were obtained.The imaging-based correlation velocimetry was proved to be a simple and effective tool for the velocity measurement of the dense gas-liquid flow very close to the nozzle exit.