Synthetic Aperture Radar(SAR) works all day and all weather, and has a wide application in the field of earth remote sensing. Video SAR expands the spatial dimension information obtained by SAR imaging to the space-time dimension to obtain richer remote sensing information. The low frequency band of traditional SAR leads to long synthetic aperture time, large amount of data calculation, and great difficulty in high frame rate output. However, the low frequency terahertz wave has strong perception on target details and short synthetic aperture, which is especially suitable for video perception of weak targets. A video SAR system working in the W-band is designed, which adopts the solid-state front end system of bistatic continuous wave, with peak output power of 1 W and maximum transmission bandwidth of 1 GHz. The Polar Format Algorithm(PFA) along with Graphics Processing Unit(GPU) processing architecture is employed to realize high frame rate and low delay imaging. The simulation results show that the imaging resolution of the system can reach 0.15 m and the imaging frame rate is 5 Hz.