Abstract: Hypersonic low-density wind tunnel test plays an important role in the study of aerodynamic characteristics of hypersonic vehicles. Velocity is one of the most crucial parameters in hypersonic low-density wind tunnel. While obtaining the flow velocity accurately by conventional velocimetry techniques such as LDV and PIV is particularly difficult since the extremely low density and hypersonic velocity in hypersonic low-density flow. Femtosecond laser electronic excitation tagging (FLEET) velocimetry technique offers an opportunity to overcome this problem. As an unseeded and nonintrusive molecular tagging velocimetry method, FLEET directly probing molecular nitrogen (N₂) instead of relying on tracer particles for velocity measurement, thereby immediately avoiding issues with particle lag and non-uniform seeding density. This dissertation seeks to answer the practical question as what measurement performance can be expected of FLEET in different pressures. It turns out that the width of optical filament’s center gradually broadening as the pressure decreases, and the intensity of FLEET signals is strong enough for velocity measurement until the pressure is as low as 90 Pa. This indicates that FLEET is well suited for velocity measurement in low density flow. Subsequently, FLEET velocity measurement experiments are conducted in Φ0.3 m hypersonic low-density wind tunnel in both Ma5.0 and Ma16.0 flow. The results suggest that as the delay time increases, the width of optical filament’s center keep broadening, and the fluorescence signal intensity gradually decreases. In contrast to Ma5.0, the fluorescence signal intensity reduced slower and the width of optical filament’s center are wider in Ma16.0. And compared with velocities measured by pitot tube, the maximum relative deviation measured by FLEET is 0.31% in Ma5.0, while which is 0.49% in Ma16.0. On the whole, FLEET as a relatively recent velocity measurement technique, has been demonstrated as an effective velocimetry method for hypersonic and low-density flow.