Fly-by-wire flight control system is more and more applied in modern airplanes. It's easier to integrate multi-functions andeasier to apply active control technology in contrast with mechanical control system. Rudder loop drives the rudder by control command. The rudder loop control characteristic is essential to the handling quality. A math model is established for the rudder servo actuator system of a civil aircraft in this paper. The frequency characteristic is studied by simulation. The simulation result is verified by the iron-bird test bench.