Abstract: A new stepped horn circularly polarized antenna uses single-arm spiral outer-fed is designed.The antenna configuration consists of a ladder horn，a single-arm spiral，and a circularly backed cavity.Single-arm spiral out-fed by air coaxial line，the proposed contains broadband characteristics in terms of the axial ratio，voltage standing wave ratio（VSWR)，and gain performances for antenna element.rotate reversed-phase feeding technique is used to improve the performance of sub-antenna such as axial ratio and polarization isolation，and solved difficult problem of narrowness space for feed network.The antenna array are fabricated and measured.Simulated and measured result show good agreements.Results show that the antenna array have substantially better performances，in terms of gain，VSWR bandwidth，axial-ratio bandwidth and radiation pattern，that have good engineering application foreground.