A freewake model is developed to analyze the aerodynamic characteristics of coaxial rotors system based on the timemar c h ing method, and the aerodynamic characteristics of co axial r otors operating in ground effect in hover are studied by using the surface panel method. WeissingerL lifting surface mo d el, which is included in the freewake model is used to simulate the blade thre edim e nsional effect considering the aerodynamic interference between the rotor/rotor and roto r/ ground. The algorithm is verified by comparing the simulational results with t est data of wake geometry and induced velocity values. Then, the wake geometry of coaxial rotors, the induced velocity distribution and aerodynamic load distribution of c oaxial rotors in ground effect are calculated and analyzed. The results show tha t the strong aerodynamic interference exists between the upper and lower rotors i n hover, and the phenomena, which are the rotor wake geometry expand rapidly in radial direction and roll up in ground effect, influence the thrust o f the lower rotor obviou sly.