The invention relates to a terahertz quasi-optical power combining and amplifying device and is applied to power combination and amplification of a terahertz frequency band. The device comprises a microwave transmission horn antenna, a dielectric lens A, a microwave amplification array chip, a terahertz frequency-doubling array chip, a dielectric lens B and a terahertz horn reception antenna. According to the focusing effect of the dielectric lens A, spherical electromagnetic wave signals input by the microwave transmission horn antenna are converted into plane wave signals which are transmitted to the microwave amplification array chip to realize amplification and output. Amplified microwave signals are transmitted to the terahertz frequency-doubling array chip to realize frequency doubling and output. The frequency-doubling terahertz signals combine into high-power signals through the dielectric lens B in the form of quasi-optical wave beams, thereby realizing coupling and matching with the terahertz horn reception antenna. The high-power terahertz signals can be outputted by adopting a quasi-optical power combining structure, thus the terahertz quasi-optical power combining and amplifying device is simple in structure and convenient to debug.