LAndscape ProcesS modeling atmUlti dimensions and scaleS (LAPSUS) model is one of the models that can calculate the multiple flow direction routing. In this model, according to the topography of the watershed, a constant value between 1 and 10 is defined as the convergence factor for the entire studied watershed. The value of this factor is considered constant through calibration or the values considered in previous studies. In this research, to solve the problem of constant convergence factor value, the flow distribution function was defined for this model. This study was conducted in five sub-catchments in the Kakhk experimental watershed located in the northeast of Iran in Gonabad county. In this research, the flow distribution function was defined instead of the convergence factor for the LAPSUS model. In this function, the slope parameter is used to model the effect of topographic characteristics on the flow distribution. In this way, the flow trending part in the LAPSUS model was developed by changing its programmed code in the C++ environment for the flow distribution function. Based on the calibration results, the best value of the convergence factor can be between 1 and 10. After applying the flow distribution function to the model, the value of the flow distribution function was calculated to be 6.3 for sub-catchment 4 and between 5.7 and 6.5 for other sub-catchments. The results of validation of the model outputs showed that the value of the coefficient of determination is 0.99 for runoff volume and 0.92 for erosion and sediment. Also, the amount of normalized RMSE was 3.5% for runoff and 5.5% for erosion and sediment. Based on the results, the flow distribution function can define the value of the convergence factor based on the topographic slope for the LAPSUS model. The value of the convergence factor can be defined for the model by entering the average slope of the watershed. Therefore, one of the main issues in the multiple flow direction routing was solved.