Thermotropic liquid crystalline polymer is regarded as the preferred material in micro injection molding industry because of its excellent properties and widely used, but the fundamental research cannot keep up with the demand of application. Aiming at the difficulty in obtaining the viscous coefficient of thermotropic materials, two numerical methods were proposed to obtain the Leslie’s viscous coefficients. The methods were based on Maier-Saupe and Doi-Edwards theory respectively. The micro-injection molding process was simulated numerically using the typical thermotropic material PolyesterX7G, and the viscous coefficient obtaining process was embedded in the simulation. By comparing with the experimental results, the method based on Maier-Saupe theory is more accurate. The conclusions enrich the fundamental research results and provide reference for wider application of thermotropic liquid crystalline materials.