Abstract: Based on microwave net theory, a cold test simulation model of double-gap output circuit with double coupling apertures was developed from that of output circuit with single coupling aperture. A computation method for gap impedance was also developed based on this model, by which the real part and imaginary part of the gap impedance can be both obtained. Meanwhile, a double-gap cavity with double coupling apertures for an S band klystron was simulated to show how to compute the gap impedance and external quality factor of the output cavity with this simulation method, and the computed result is verified to be reasonable in theory. The computed result was analyzed and compared with those obtained with the field analysis method and the scattering parameter of output circuit, confirming the reliability of this cold test simulation method.