Abstract: In order to reduce the adjacent channel interference and car following interference in the current electronic toll collection ETC system, a 5.835 GHz microstrip array antenna is designed. In this paper, the circular polarization of the antenna is realized by cutting the square patch angle. To improve the axial ratio bandwidth, four antenna elements are compensated by means of rotation and phase compensation. The improved antenna elements are used as the radiation elements of the microstrip array antenna. In terms of low sidelobe and high orientation, the paper designs a feed network with unequal amplitude feeding based on the Dolf-Chebyshev method of amplitude distribution. Through a large number of electromagnetic simulation, the optimal structure of the antenna is finally determined, and the physical processing and measurement are carried out. The measured results show that the impedance bandwidth is 5.67−5.88 GHz, the gain is greater than 15 dB, the axial ratio is less than 3 dB, and the E-plane half power lobe width is less than 12 degrees in the 5.7−5.9 GHz frequency band. The measured results are in good agreement with the simulation results. The antenna has the characteristics of low sidelobe, circular polarization and high orientation, which provides a novel antenna structure for the roadside unit array antenna of electronic toll collection system.