An integrated, high-precision time-of-flight (TOF) readout circuit for terahertz (THz) radar was designed to perform high-precision TOF measurements of THz radar in target detection, imaging, and other applications. Based on an in-depth analysis of the TOF readout mechanism of THz radar, a high-speed comparator circuit, time-to-digital converter circuit, and field-programmable gate array circuit were designed to realize the effective digitization of THz detector response signals, accurate time-to-digital conversion of TOF information, and efficient control and data transmission of readout circuits. A high-precision arbitrary waveform generator was utilized to calibrate the readout circuit and verify its linearity, accuracy, and stability. The experimental results show that the present readout circuit exhibits excellent linearity and stability, with a resolution of 45 ps over the measurement range of 0.1 μs to 1.5 μs and a TOF measurement accuracy that is better than 40 ps, thus satisfying the application requirements of THz radar.