Abstract: Phase noise is one of the key specification in the application of terahertz(THz)wave technology,how to accurately measure the phase noise of terahertz wave is an important research direction of terahertz technology.In this paper,the （110~500）GHz terahertz phase noise measurement system is built,which is based on frequency expansion by using the principle of terahertz down conversion.The experimental results show that the measurement system has expected noise floor and can be used to measure the phase noise of terahertz frequency source in the frequency range of （110~500）GHz.It has a broad application prospect in the measurement and testing of terahertz phase noise,standard development and related product transformation.

Key words: Terahertz electromagnetic wave, Phase noise, Harmonic mixing, Down conversion