When runoff yield excess was calculated by using Horton infiltration formula, and the corresponding infiltration capacity was calculated based on watershed soil moisture (W) by using iterative algorithm, it was found that when W is lesser, iterative calculation is normal; when W is moderate, iterative oscillation often occurs; when W is larger, iterative calculation error is easy to happen. This paper, therefore, analyzed the reasons of the iterative oscillation and iterative calculation error, and proposed to re - ducing iteration oscillation by using arithmetic mean method and solving the problem of iterative calculation error by combining the restriction of oscillation amplitude with arithmetic average method. Meanwhile, the experience relation formula of watershed average water storage capacity and the three parameters in the Horton infiltration formula was deduced in the mode of runoff yield excess.