Terahertz Video Synthetic Aperture Radar (ViSAR) system can indicate target information continuously and actively with an image frame rate as high as infrared video, having advantages such as high resolution and good sensitivity in target motion detection. However, terahertz wavelength is rather short, and as a result, traditional dual-channel SAR motion target indication method gets a tiny blind velocity period which goes against target detection. Aimed at the problem, this paper provides a multi-channel terahertz ViSAR motion target detection method based on along track interferometry (ATI) technique. It selects several baselines of varying length and utilizes algebra coprime theory to enlarge blind velocity period so that the range of non-ambiguity velocity can be extended. Simulation results verify the effectiveness of the method.