Based on the transmission terahertz time domain spectral system, a novel rotating optical delay line is designed. The rotating optical delay line contains 24 planar mirrors, which can change the optical path by rotating to produce a variable time delay. A mathematical model of the optical delay line is established, and the optical path simulation and the actual experimental verification under working conditions are carried out. Simulation and experimental results show that the delay time can reach 72 ps, the scanning frequency is 60 Hz, and the linearity calculated by the least square method can reach 99.96%. Using the finite element analysis software, the force and modal analysis of the mechanical structure of the delay line are carried out in the rotating state, which ensure the reliability and stability of the design in work. Compared with the linear delay line in the existing terahertz time domain system, this FRODL has a fast scanning speed and linear delay time. The FRODL is more suitable for fiber optical systems, which has great advantages of miniaturization and integration of the THZ-TDS system relative to linear optical delay lines.