Optical add-drop multiplexer is one of the promising components for integrated optics. For the existence of loss at the cross of optical add-drop multiplexer, the influence of the insertion loss on the gap between channel and microring over the output spectrum is introduced. Based on mode coupling theory, the analytical expressions are derived. The output spectra are simulated and compared. The simulation results show that the cross coupling coefficient is the smallest when the gap is 66.16 nm. The spectrum exhibits an asymmetric Fano-resonance line shape because of the existence of the cross coupling. The finite difference time domain (FDTD) simulation results agree well with the analytical results from the comparison between FDTD and analytical results.