Balanced detector, which is located at the end of high-speed Terahertz Time-Domain Spectroscopy(THz-TDS) system, is a device to detect THz signals directly. The optical signal detection part is the key device for converting the optical signal into analog electrical signal. Its performance determines the accuracy of the measured THz signal. Using two low-noise photodiodes in series, the differential photocurrent of two lasers is directly detected, and the detection noise is reduced. The operational amplifier with a gain bandwidth product of 4 GHz is utilized to perform two-stage amplification of the differential photocurrent. The THz signal with high dynamic range is obtained and the detection speed of the THz signal is improved. The absorption peaks of α-lactose monohydrate are measured, and the same absorption peaks as those in literatures are obtained.