The invention discloses a three-frequency micro-strip antenna, and belongs to the technical field of antennas. The three-frequency micro-strip antenna comprises a dielectric substrate, a radiation patch, and a ground plate. The dielectric substrate is arranged between the radiation patch and the ground plate, the radiation patch and the ground plate are arranged on the dielectric substrate, and the lower end of the radiation patch is connected with the lower edge of the dielectric substrate. The ground plate includes a rectangular plate and an arched plate. The lower end of the rectangular plate is connected with the lower edge of the dielectric substrate, the upper end of the rectangular plate is connected with the arched plate, and the arched side of the arched plate is away from the rectangular plate. The rectangular plate is provided with a first defect, and the arched side of the arched plate is provided with a second defect. Through the first defect and the second defect, a defected ground structure is formed on the ground plate so as to excite and improve the resonant frequency of the three-frequency micro-strip antenna and increase the impedance bandwidth.