The invention relates to a terahertz quasi-optical power combining and amplifying device. The terahertz quasi-optical power combining and amplifying device comprises a microwave emitting horn antenna, a main parabolic reflector group A, a secondary parabolic reflector group A', a microwave amplifying array chip, a terahertz frequency doubling array chip, a main parabolic reflector group B, a secondary parabolic reflector group B' and a superheterodyne terahertz quasi-optical receiver. The main parabolic reflector group A and the secondary parabolic reflector group A' are used for converting a space-feeding spherical electromagnetic wave signal to a plane wave signal and transmitting to the microwave amplifying array chip to amplify and output. The amplified microwave signal is transmitted to the terahertz frequency doubling array chip to realize the frequency doubling and output. After frequency doubling, the terahertz signal combines with a big power signal in the space by the form of beam focusing through structures of the main parabolic reflector group B and the secondary parabolic reflector group B', and the terahertz signal is received by the terahertz quasi-optical receiver. By using the quasi-optical power combining structure, the high-power terahertz signal can be output; the terahertz quasi-optical power combining and amplifying device is simple in structure and convenient to debug.