An Archimedean spiral antenna was designed for global navigation satellite system (GNSS), which included GPS, GLONASS and BD.The proposed antenna had the wide band covered from 1.2 GHz to 2.6 GHz.The balun was improved to lower the profile. Moreover, the PEC backed cavity was used to obtain a unidirectional beam. The ring slot was designed to improve the VSWR and the axial ratio. The antenna was fabricated and the measured results showed that the antenna had a gain greater than 3 dBi;the VSWR and the axial ratio were separately less than 1.8 and 4 dB.