Abstract: In the paper, the heat dissipation of high power microwave tubes collector is calculated by using Euler two-phase model and the boiling model. Based on the electron beam sink rule, the distribution of heat source in collector material and cooling water is obtained. Then, the collector temperature is calculated by using the CFD software under electron beam duration of 45 ns, a repetitive frequency of 50 Hz and an average power of 27 kW. The effect of flow velocity of the cooling water on heat dissipation is analyzed. The results indicate that the max temperature of the collector is beyond the material melting point when the cooling water flow velocity is 1.5 m/s, and the collector will be damaged after some time. At the top of the heat dissipation, the convection heat transfer is about 71.7% of the total heat transfer. The quenching heat transfer is 28.1%, and the phase transition heat transfer is 0.2%. When the cooling water flow velocity is less than 5 m/s or more than 10 m/s, the maximum temperature of the collector will decrease quickly as the flow velocity increases; when the cooling water flow velocity is 5-10 m/s, the temperature will decrease slowly as the flow velocity increases. For the electron beam condition in the paper, it is requested that the cooling water flow velocity is 5 m/s at least for collector safety.