Slot coating is a kind of precise coating process, which is widely used in the production of membrane materials. The thickness deviation of coating along the transverse direction is the key index to measure the coating quality. The homogeneity of the transverse distribution of the coating velocity at the exit of the die is the most direct factor leading to the transverse thickness deviation of the coating. To predict the transverse homogeneity of slot coating, a mathematical model for the internal flow field in the die was established, where the incompressible Navier-Stokes equations were used to describe the fluid flow and generalized Newtonian fluid model was used to describe the rheological behaviour of non-Newtonian coating. An efficient explicit splitting algorithm based on finite element method is proposed to solve the model. To verify the effectiveness of the model and the algorithm, the internal flow field of a trapezoidal double cavity die with equal section was simulated. The transverse homogeneity obtained was close to the experimental measured value, which verified the validity of the model. Furthermore, the influence of measuring slot width on the transverse homogeneity of outlet velocity is studied, and the simulation results are consistent with the theoretical expectations.