To study the specialties of wave up-lift force on horizontal plate structures,we establish two-dimensional regular wave numerical models based on the software FLUENT. RANS equations and standard k-ε equations are adopted and VOF method is used to reconstruct the free surface. Firstly,three typical cases of wave slamming on horizontal plate structures are simulated. Using the low-pass filter method,we obtain the slowly-varying sections from the up-lift force signals. Based on the comparison of the simulated and experimented results,we verify the effectiveness of the model. We also discuss the influence of wave steepness,structure geometry and the distance between underside of horizontal plate and still water level on the maximum slowly-varying force. Finally,the empirical slowly-varying force formulas are provided.