Abstract: A coaxial hybrid wiggler free-electron laser(FEL) amplifier with and without plasma filling is studied by a method of PIC simulation in this paper. The results show that signal in FEL amplifier with a coaxial hybrid wiggler can get an efficient amplification even at a low voltage of 205 kV. The efficiency of the amplifier is upto 27.5% and the saturation power arrives to a MW level. Plasma has some beneficial effects on the characteristics of the FEL amplifier, for example, it can improve the characteristic of electron beam transmission.