In this paper, based on computational fluid dynamics (CFD), the simulation study was conducted on the thermosyphon to construct the mathematical configuration of the thermosiphon. Then the CFD data simulation was performed for the thermosiphon and also the experimental comparison was made. The experimental results show that the thermosiphon has the best heat conduction effect under the condition of 500W energy supply and FR=0.4 filling rate. According to the simulation results, the increase of the FR has two different functions. First, the fluid volume becomes larger, and the boiling heat conduction effect can be deepened. Second, the liquid filling effect shall be enhanced and then the liquid surface be strengthened, which will cause many bubbles in the liquid phase of the evaporation zone and thus reduce the thermal conductivity; the most important is that with the liquid-filled effect increased, the gas phase area shall be increased and the pressure will be larger, so as to reduce the steam flow efficiency and overall system efficiency.