In order to provide testing support for the development and production of terahertz instruments such as terahertz source and terahertz detector, a terahertz power measuring instrument based on phase-locked amplification principle is developed. At present, many commercial phase-locked amplifiers are expensive, bulky and complex in structure and function, which are not suitable for some portable detection systems. The instrument mainly uses a phase-locked amplification scheme based on AD630 to monitor weak signals. AD630 phase-locked amplifier has low cost, simple design structure, strong flexibility and high integration. When the terahertz power measured by the standard terahertz power meter is 96.8 mW and 2.98 W, the power measured by the terahertz power measuring instrument developed in this paper is 97.9 mW and 3 W respectively, and the error range is within ±5%. The phase-locked amplification scheme based on CD552-R3 weak signal can only extract millivolt level signals, while the scheme in this paper can extract microvolt level signals. Therefore, the proposed scheme is used to detect terahertz power with a wider range and higher stability.