There are complex rotor/rotor/ground interference effects when a coaxial-rotor helicopter hovers near ground. A computational method based on free-wake model is developed to analyze the flow field and thrust of coaxial-rotor helicopter in ground effect. Firstly, the lifting-surface blade model and the ground panel model are taken in this method to simulate the effects of blades and ground to the flow field. A 3^rd-order accuracy time-marching scheme is conducted to get the free-wake of coaxial-rotor. Also, the constant-volume-rectification method is incorporated to prevent numerical divergence. Then, the coaxial-rotor out of ground effect (OGE) and the single-rotor in ground effect (IGE) are used as examples to validate the method. Finally, based on the examples, the features are analyzed, including the characteristics of wakes and flow field, the distribution of velocity and thrust in rotor disk plane, the interference effects between two rotors and the gain of thrust. Calculated results show that the wake rolls up and expands radially in ground effect when hovering, besides the induced velocity in rotor disk plane is connected with upper rotor, lower rotor and ground. The results show that ground effect causes the more uniform distribution of induced velocity and thrust along the rotor. Besides, the gain of thrust for coaxial-rotor is higher than that of a single rotor when the power is kept the same.