This paper proposed an optimal algorithm for the calculation of urban runoff direction based on Cellular Automata(UCA). The optimal water distribution weights that is suitable for different flow patterns are selected by the flow simulation experiments on slopes. The UCA, D8, 4+4N and FMFD algorithms are used to calculate the Specific Catchment Area(SCA)on four mathematical surfaces, and the results of four algorithms are compared. The relative error(RE), root mean square error(RMSE)and mean absolute error(MAE)are chosen to evaluate the performance of four algorithms. The results showed that the optimal water distribution weights of UCA algorithm for the converge flow and the diverge flow are 2. 0 and 1. 1 respectively, and the RE and RMSE between the UCA simulated and theoretical values of SCA are the smallest on convex slopes, concave slopes, ridges and plane slopes. The UCA algorithm is suitable for the diverge flow on urban flat terrain and the converge flow on terrains with steep hydraulic gradient, which can provide a better solution for the simulation of urban runoff.