The invention discloses a dual-mode gyrotron traveling wave tube amplifier, and relates to the technology of a microwave source device. The dual-mode gyrotron traveling wave tube amplifier has two-section structure, wherein the first section is a linear amplification section which adopts loss-loading metal waveguide and works at the low step mode; and the second section is a nonlinear amplification section which adopts smooth metal waveguide works at the high step mode; the linear amplification section provides high gain for the amplifier; and the nonlinear amplification section provides highpower. The nonlinear amplification section works at the high step mode, thus having higher waveguide transverse size and stability threshold current. The dual-mode gyrotron traveling wave tube amplifier solves the problems of low working current and poor stability of a single-mode amplifier, and has higher output power and more superior stability, thus being a high-power millimeter wave source with good development potential and having development prospect in the important national defense fields such as high resolution long-range radar and the like in China.