This paper presents a new design of differential-fed dual-polarized microstrip patch filtering antenna based on dual-mode substrate integrated waveguide (SIW) cavity. The antenna consists of a dual-mode SIW cavity, four coupling slots, and four stacked patches. By exploiting the TE102 and TE201 modes within the SIW cavity, an in-band resonance is achieved, along with effective polarization isolation. The TM10 and TM01 modes within the patch radiator are utilized to generate an additional resonance and facilitate radiation. Furthermore, a radiation null is achieved by introducing a coupling path between the source and the patches. The proposed design is analyzed, designed, and simulated, resulting in a bandwidth of 5% (from 5.06 GHz to 5.32 GHz), a peak realized gain of 10.8 dBi, polarization isolation exceeding 34 dB, and a radiation null located at 4.84 GHz. These simulated results demonstrate the effectiveness of the design in achieving the desired filtering and radiation characteristics.