Multifrequency filtering characteristics of the two-dimensional square-lattice photonic crystal structures with rectangular microcavities are studied. Owing to the different sensitivities of the resonant frequencies to the variation of the rectangle’s side length, one resonant frequency can be flexibly adjusted only by changing one of the rectangle’s side lengths, while another frequency is immunizing to the variation. So, a two-output-channel frequency filter is designed to allow two frequencies to travel along different channels, and allow the third frequency to travel along both the channels. Based on the low spatial symmetry of rectangular defect, which supports localized modes with different symmetries, together with optimizing the coupling regions between the input waveguide and the rectangular microcavities, the two-output-channel and four-output-channel filters are both achieved, which can select different frequencies to travel along various channels, and share another frequency to travel along every output channel. These kinds of devices have both the abilities of information selecting and sharing carried out by different frequencies, and may have potential applications in the future complex all-optical integrated circuits.