Abstract: In order to study the effect of rotor spacing on the aerodynamic performance of small-scale coaxial rotor in hover, a platform is designed to test the aerodynamic characteristics of the coaxial rotor system with different rotor spacing ratios h/r of 0.32, 0.38, 0.45, 0.51, 0.58, 0.65 and 0.75. Rotor thrust and power obtained from measurement with different rotary speeds are used to estimate the aerodynamic characteristics of the rotor system. In the meantime, the uncertainty of the thrust coefficient, power coefficient and power loading (PL), and the calculated errors are all less than 2%. Distributions of streamline and pressure with different rotor spacing ratios are obtained from numerical simulation to analyze the interference between top and bottom rotors. Finally, in comparison with test results, the aerodynamic configuration is optimized with the rotor spacing ratio of 0.38. The results prove that the performance of coaxial rotor in hover can be improved with proper rotor spacing. With certain rotor spacing, aerodynamic performance at higher rotational speed is more prominent when the interference between the rotors is beneficial for the aerodynamics, and greater power loading is obtained at lower rotational speed with smaller interference.