Abstract: In this paper, the characteristic mode analysis method is used to suppress the out-of-band mutual coupling of two bowtie aperture antennas with center frequencies of 1 GHz（in-band） and 2 GHz（out-of-band）. The in-band coupling mode and out-of-band coupling mode of the antenna are given respectively on the basis of analyzing the characteristic mode of the antenna. By adding capacitive loadings on the bowtie aperture, the out-of-band coupling mode of the antenna is suppressed, while the in-band coupling mode is retained. The simulation and test results show that the out-of-band coupling of the antenna at 2 GHz is reduced by about 24.5 dB while the S parameter and far-field radiation pattern have little change at 1 GHz after loading the capacitive loads, which indicate the in-band performance of the antenna is retained. In addition, this paper uses the same method to suppress the out-of-band mutual coupling of the linear arranged and the two-dimensional three-unit bowtie aperture antennas system. Base on the simulation, the out-of-band mutual coupling at 2 GHz is reduced at least 18 dB by loading capacitive loads.