In present terahertz spectrum measurement, the commercial lock-in amplifier for signal detection has disadvantages such as function redundancy, bulky size and high price. A simplified lock-in amplifier circuit was designed in view of these disadvantages. The circuit uses a STM32 controller based on ARM Cortex-M3 generating chopper signal and reference signal with programmable relative phase, driving a balanced modulator/demodulator to realize lock-in amplification. A test system for current mode lock-in amplifiers was proposed based on the Norton’s theorem to evaluate the performance of the designed lock-in amplifier. Test results show that the maximum relative error of designed lock-in amplifier is only 0.56%, and the dynamic range reaches up to 84.2dB, which means that the designed circuit meets the requirement in THz-TDS system. The size of the designed lock-in amplifier circuit is only 85×85 mm2. Through a RS232 port, the reference signal phase, system gain and time constant can be programmed and the data acquisition can also be completed, which is convenient for integration and application for THz-TDS system .