The design of small watershed flood peak flow inference formula involves transcendental equation solving, and the present iterative method and the simplified formula exist complex calculation process and poor solution accuracy, and the use of computer to solve is inconvenient work. Combined the advantages of Newton iterative method and simple iterative method, the paper settled a theoretical analysis formula and fitted a theoretical formula by optimized fitting method and obtained were alternative forms of expression is simple, calculation convenient, to facilitate the practical application of the approximate analytical style. Accuracy analysis shows that to the maximum relative error calculated by the formula is less than 0.42%. An example shows that to use this formula to complete the design of small watershed peak flow calculation can greatly simplify the calculation process, improve significantly the working efficiency and has practical promotion significance.