The invention provides a terahertz quantum cascaded optical amplifier comprising a semi-insulating GaAs substrate; a GaAs buffer layer on the surface of the substrate; a n-type heavily doped lower contact layer on the surface of the buffer layer; an active region on the surface of the lower contact layer; a n-type heavily doped upper contact layer on the upper surface of the active region; mutually-separated upper electrode metallic layers on the surface of the upper contact layer, wherein deep isolating trenches are arranged between adjacent upper electrode metallic layers and are recessed to the buffer layer; and a lower electrode metallic layer on the surface of the lower contact layer and both sides of the active region. The optical amplifier, according to the upper electrode metallic layers, comprises a main waveguide section and at least two coupled cavity waveguide sections on both ends of the main waveguide section and distributed symmetrically. The main waveguide section and the coupled cavity waveguide sections are arranged along the same straight line in the horizontal direction. The optical amplifier solves a problem that a conventional optical amplifier is formed by discrete elements, cannot be prepared with a pre-stage THz QCL simultaneously to achieve on-chip integration, and is required to be subjected to optical alignment in a used process.