Abstract: The complicated flows around the ducted co-axial rotors and the ducted isolated rotor were numerically simulated by steady Navier-Stokes(N-S)equations based no unstructured grid,and the aerodynamic characteristics between them were compared.The computational domain was decomposed into subzones.The lower-pressure area at the leading duct leads to the generation of additional thrust on the duct.The thrust performances of two rotors became worse because of the interaction between them,and the influence of the upper rotor on the lower rotor was greater than that of the lower rotor on the upper rotor.The downwash velocity and thrust coefficient became larger when the collective angle of blades was added.With the increase of distance between the co-axial rotors,the thrust coefficient of the upper rotor and the lower rotor increased,the total thrust coefficient increased,and the thrust performances of two rotors became better because of the interaction between them.With the increase of the collective angle of blades,the additional thrust increased which generated on the duct of ducted co-axial rotors and single rotor,the additional thrust on the duct in the total of the proportion of thrust reduced.With the increase of the distance between duct and rotor,the total thrust of ducted co-axial rotors will reduce.Hence,we should reduce the distance between duct and rotor as much as possible during the process of our design.