Based on the Blade Element Momentum Theory （BEMT）， the inflow distribution of the rotor of a coaxial rotor system was determined and the aerodynamic model for these coaxial rotors was established in this study. Next， the aerodynamic characteristics （thrust， torque and power consumption） of the coaxial rotor system were measured and verified by the designed test platform. To explore the optimal aerodynamic layout， the influence of varying the rotor spacing on the system performance in a hover at different rotor speeds was emphatically analyzed. The results show that varying the rotor spacing has no significant effect on the total thrust in the coaxial system， however， the lower rotor produces a smaller thrust than the upper rotor， which is approximately 43.8%-45.1% of the total thrust. In conclusion， improving the collective pitch and blade twist angles of the lower rotor aids in enhancing the aerodynamic performance in the coaxial rotor system. Moreover， the area of the actual wake boundary of the upper rotor is approximately 15% larger than that of the theoretical boundary as the lower rotor greatly restrains the contraction of the upper rotor's wake. In addition， the aerodynamic layout is optimal when the rotor spacing is 0.40R owing to the largest thrust produced by the lower rotor and the lowest net torque.