The axial-flow impulse turbine is recently utilized for oscillating water column wave energy conversion because of its wider operating range and having no stalling point. Most previous experimental studies conducted in the oscillating air flow testing rigs can give little information of the steady-state performance of impulse turbines. A constant flow test rig was set up in this study. The steady state performance of an axial-flow impulse turbine was studied by carrying out over 140 cases. The total pressures at the chamber and atmosphere sides and torque output under various incident flow velocities and prescribed rotation speeds are presented. Effects of Reynolds number on the dimensionless coefficients are reported, which are also compared by a previous study. The scatter diagram of efficiencies provides detailed information of turbine performance under different testing conditions. The peak value of turbine efficiency is found to be 0.48 at the flow coefficient of 1.0.