Abstract: To describe the attachment vortex circulation distribution of blades, contacting blade element theory, trigonometric series was adopted and applied for the model of classical rotor vortex theory. Some studies have been done for the air-operated characteristic of single rotor and co-axial rotors in hover. The attachment vortex circulation distribution of blades which is continuous and smooth was obtained by rough computation. While it is applied to the formulas found in the page, the induced velocity of continuous distribution can be resolved. Then analysis was done for the complex interaction between the upper and lower rotor, and the condition of balance between the torque of upper and lower rotor was got. The comparison between calculation results and experiment records shows that the previous theory and method is applicable. There are two conclusions: One is that while collective pitch of lower-rotor is changed, sharp influence is brought to lower-rotor, but it is nearly no influence for upper-rotor, the other is that when torque of upper-rotor equals to lower-rotor', the collective pitch of lower rotor is greater 1° than upper-rotor'.