Abstract: The AC noise in the intercommunication system is a problem of the Z-9 helicopters, with the phenomenon of boring noise in the headphone. In order to solve the problem, the wires of the power and the voice in the intercommunication system are analyzed, and the quantity of resistor coupling, capacitive coupling and inductive coupling are calculated. The methods of isolation and reducing the sharing resistance are used to solve the problem. The result shows that the methods can solve the problem effectively. It is pointed out that the major interferential path of lower frequency circuit is the resistor coupling and the inductive coupling and that electromagnetic interference should be reduced by isolation wires and decreasing the sharing resistance in the design.