The optimization function of high viscosity gear pump and the corresponding constraint requirements were determined. MATLAB was used to optimize the parameters, and the whole volume of the optimized gear pump was reduced by 23.99%, so the manufacturing cost of the high viscosity gear pump was reduced. Based on the optimized gear pump parameters, the high viscosity medium with viscosity value of 1.076 48 Pa·s was selected, the temperature and speed were set as variables to simulate the internal flow field of the gear pump, especially the flow field at the meshing of the gear pump. The simulation results show that the higher the viscosity is,the smaller the effect of rotating speed on the volume efficiency is; the different temperature can affect the distribution of high viscosity medium in the gear pump and the volume efficiency of the gear pump. It provides reference for the application of high viscosity gear pump in chemical industry and food industry.