To measure the velocity distribution of a complex flow， a femtosecond laser electronic excitation tagging （FLEET） system was established based on the principle of molecular tagging velocimetry. The velocity distribution of a supersonic mixing layer flow， which was generated from a primary stream of Mach number 3.0 and secondary streams of Mach numbers 2， 2.5， and 2.9， were obtained. In addition， the flow structure of the mixing layer was obtained through large-eddy simulation and a schlieren experiment. The uncertainty of FLEET velocity measurement is better than 5 m/s by calculating the displacement difference between the fluorescence baseline and the 10-μs-delay fluorescence filament. In the main stream， the velocity results measured by FLEET were essentially consistent with the calculation results. In the mixing layer， the obvious gradient velocity distribution was obtained， and the thickness of the mixed layer was essentially consistent with the schlieren experimental result. Therefore， the FLEET system can be used for complex velocity distribution measurements， such as supersonic mixing layer flow.