Abstract: Ultra-high vacuum (UHV) has been demonstrated as the most important condition for long lifetime GaAs photocathode. To create the UHV environment and extend the GaAs lifetime, the electron beam wall loss in DC injectors is studied in this paper. A DC injector is installed in free electron. laser THz (FEL-THz) facility in CAEP. The study is based on Gaussian distribution model. With theoretical analysis, numerical calculation, dynamic simulation, thermal simulation and prototype experiment discussion, this beam loss is proved to be a few watts, indicating that it is the main reason for vacuum and quantum efficiency loss, which will dramatically decline the beam-out time. Furthermore, the study also indicates that the tube diameter the after the anode should be at least 45 mm.