The work aims to design a double-head flat-push plunger filling mechanism to improve the filling and production efficiency of the viscous pastes, such as toothpaste and study the effect of the rheological characteristics of toothpaste on filling thrust at the same time, so as to solve the current problems of low filling speed and accuracy of toothpaste and other viscous pastes. Firstly, the toothpaste with high market share was selected for the rheological test, and then the rheological curve was fitted by the classical non-Newtonian fluid constitutive equation. Finally, the filling and extrusion process of material was simulated to obtain the maximum filling thrust required. According to the study results, toothpaste was a non-Newtonian fluid and had rheological properties of shear thinning. The correlation coefficient between the toothpaste and the power-law rheological model (Powder law viscosity model) was 0.999 2. At the maximum filling capacity of 200 mL and the maximum filling speed of 0.337 m/s, the required thrust was 1 681 N. The conclusion can provide theoretical guidance for the filling mechanism design of non-Newtonian fluid.