In order to solve the problems of narrow impedance bandwidth and axis ratio bandwidth of traditional single-fed single-layer microstrip antenna, a method to improve the gain, standing wave and circularly polarized bandwidth of probe fed single-layer circularly polarized microstrip antenna is proposed. By loading a square ring metal patch above the radiation patch based on the principle of circular polarization radiation of annular patch, the inner and outer rings of the patch are equidistant from the edge of the radiation patch, which increases the impedance and axial ratio bandwidth, improves the antenna gain, realizes low profile application. The measured results show that The bandwidth of VSWR＜2 is 17.2 % (370 MHz ~ 440 MHz), and the 3-dB axial-ratio bandwidth is 8.4 % (395 MHz ~ 430 MHz). The gain is about 7 dB within the overlapped bandwidth. This method is of great significance to improve the light and miniaturization and broadband technology level of single-fed circularly polarized microstrip antenna, and has broad application prospect.