Abstract: The beam dump is the primary assembly used at the end of the accelerator to absorb high power electrons. In this paper, a beam dump was designed for 8 MV/5 mA CW electron beam of the terahertz free electron laser (FEL-THz) facility in China Academy of Engineering Physics, whose inner surface absorbed average power is up to 40 kW. The beam dump design was based on a 1.2 m long copper cone whose inner surface absorbs the beam. The beam dump cooling structure and flow rate were optimized, so that the beam dump temperature was controlled at 355 K, to meet the requirements of high average power FEL-THz facility.