According to low heat dissipation efficiency of heat dissipation pipe, building a 3D model of heat dissipation pipe with Solidworks, the boundary conditions enacted under actual experimental conditions are stressed outside the model. The outlet temperature of the heating-radiator under the condition of special heat dissipation pipe size is obtained through finite element analysis on the pipe model using ANSYS thermal fluid coupled module. The new outlet temperature is obtained through adjusting the pipe size. The data group of the outlet temperature is obtained through adjusting the pipe size many times. Finally, according to the data group after analyzing, the pipe size is adjusted using bisection method to realize the expected target of heat dissipation pipe size optimization. And it provides references for the similar size optimization design in the future.