Using differential circuit constituted by two photodiodes to detect terahertz (THz) wave is the most basic detection method in THz time-domain spectroscopy system. Before detection of THz wave, the first task is to adjust the quarter-wave plate and make the polarized light beam of equal intensity when femtosecond detection light goes through the Wollaston prism, that is to say that the output differential current of the photodiodes is zero. At present, the most common method of adjusting the quarter-wave plate is manual regulation through the observation with the naked eyes by virtue of the experience of operators, so the regulation accuracy is very low. An autobalance device which can make the s polarized light and p polarized light generated by Wollaston prism have the same intensity is developed. The operational amplifier circuit, voltage conversion circuit and analogue / digital (A/D) conversion circuit are used to process the differential signaling of the two photodiodes, the single chip microcomputer is adopted to control rotation of the quarter-wave plate driven by a stepping motor, and the optical balance can be reached fast. The balance precision of 0.9° can be achieved by the developed autobalance device.