Abstract: By connecting the scattering matrices of feeding networks and antennas together, methods on calculating the scattering parameter at the input port of antenna array and the actual excitation distribution of antennas are presented with considering the coupling effect. Based on the method, a novel fast method for designing slotted waveguide antennas is proposed in this paper, and a 12-element linear array with longitudinal slots in the broad wall is designed. The measured results show that the reflection coefficients and the side-lobe levels are better than -15 dB and -28 dB from 9.82 GHz to 10.21 GHz, respectively. The measured results accord with the computed and simulated results well, which demonstrates the accuracy and efficiency of the proposed method in this paper.