An optimum method is presented to obtain the track parameters of wire electrical-discharge machining (WEDM) of closed interface on the ring with high precision. Firstly, the three-dimensional model in WEDM motion space of the ring was obtained based on AutoCAD software and WEDM experiment. Seven track parameters were defined, and then the effects of the position size L1 of P1 space along the direction Z upon six other parameters were discussed in the paper. Finally, the constrained condition about the track parameters of wire electrical-discharge machining was analyzed and the relations between the track parameters and fit characteristics of the joint were studied. The results show that the six other track parameters change evidently, fit stability of the interface is reduced, and inosculation degree is increased when L1 increases from 10.27mm to 28.27mm for Φ105 ring. It is concluded that the optimum method and track parameters can be applied in the design and manufacture of ring under the condition of L1 limited from 16.16mm to 19.76mm.