We present a terahertz (THz) real-time digital holographic system with zoom function worked at 0.17 THz.The magnificationfactor ranges from 1 to 2.In the imaging experiment,the resolution is 2 mm with the magnification factor of 1.2.A metal sheet with F-shaped hollow is used as a sample,and its THz holograms are reconstructed by our developed algorithm based on the angular spectrum theorem,and the qualities of the THz images under different conditions are compared.