Abstract: A kind of Sband frequency agile relativistic magnetron is designed, which adopts the scheme of risingsun magnetron with 5 fanshaped cavities and 5 rectangular cavities. Through adjusting the site of metallic block in the rectangular cavities, the system's syntonic frequency gets altered. The method of equivalent circuit, the software of highfrequency structure simulator (HFSS) and MAGIC are used to analyse and simulate this device. Initial results show that 20% relative bandwidth, minimum output power 600MW can be gotten in this frequency agile relativistic magnetron, when operating voltage is 650kV with 0.1ns rise time and magnet field of 0.6T respectively.