The invention discloses a low-RCS microstrip patch antenna based on polarization conversion. The low-RCS microstrip patch antenna based on polarization conversion is used for solving the defect of poor scattering performance of existing microstrip patch antennas. The low-RCS microstrip patch antenna based on polarization conversion comprises a radiating unit (1), a dielectric slab (2), a metal ground slab (3), a coaxial joint (4) and a polarization conversion surface (5). The upper surface and the lower surface of the dielectric slab (2) are printed with the radiating unit (1), and the polarization conversion surface (5) as well as the metal ground slab (3) respectively; the polarization conversion surface (5) is composed of four polarization conversion unit groups (51), and the center of the polarization conversion surface (5) is provided with a square area (6); every polarization conversion unit group (51) comprises a plurality of polarization conversion units (511) with the symmetric axises provided with square notches (512) which point to the same direction. The low-RCS microstrip patch antenna based on polarization conversion has the advantages of being stable in radiating performance and high in scattering performance and can serve as a communication antenna on a low-RCS device.