The design of an spiral antenna was theoretically analyzed, and a high frequency broadband K band spiral antenna was realized by Archimedes spiral antenna with lighter quality, lower profile and better circular polarization Finally, an Archimedes spiral antenna cover the band from 18.0 GHz to 26.5 GHz was designed through simulation software CST , covering the entire K band. The results showed that the voltage standing wave ratio of the antenna was less than 2 in the whole frequency band, which could well meet the requirement of transmit receive of an antenna.