Abstract: The polarization mismatch caused by variation of signal polarization angle and base gesture often leads to accuracy decrease in high power microwave(HPM) radiation precise measurement. It is commonly difficult to realize polarization match with linearly polarized antenna. Based on analysis of trapezoidal circularly polarized antenna, method of sequentially 90° rotation and phase shift with power divider and phase shifters as feedwork, a novel circularly polarized microstrip array antenna with low axial ratio feature was designed. The experimental behavior of this antenna shows its gain is 9.4 dB, 1 dB angular width is 17.4°, and axial ratio is as low as 0.4 dB. The application of the designed circularly polarized antenna is able to effectively reduce polarization mismatch uncertainty to less than 0.2 dB, which satisfies the requirement of airborne HPM radiation measurement.