Obtaining the cable tension accurately is important to the structural safety. Among different cable tension testing methods, the method of testing stress under the excitation of constant magnetic field is not affected by the boundary condition of the cable and avoids the affection of eddy current. Through analyzing the structural features of parallel wire cable, a cable tension testing method is presented based on the principle of testing stress under the excitation of constant magnetic field. An experiment setup is carried out to verify the feasibility of the method.