Unit hydrograph of surface runoff is widely used in hydrological calculation and forecast. However, when the net rainfall with multi-intervals appear, some advantages exist in the traditional approach of unit hydrograph method such as heavy, inefficient, complex, and the simulation results may be negative. In this paper, Free Search (FS), derived from the animal group migration behavior, was used as a method for estimating surface runoff unit hydrograph. The position vector of each exploratory step for the individual animal of FS was ascertained as an initial unit hydrograph. All the initial unit hydrographs were converted into the standard unit hydrographs in the iterative process of FS, and the optimal unit hydrograph was eventually ascertained from the animal group migration. FS exhibits the advantages of the clear concept and simple operation, and the flow discharge of each time interval can be given directly. The simulation results for three cases show that the satisfactory results could be achieved by FS for optimizing the unit hydrograph once; the unit hydrograph without negative and need not to be fixed; the fluctuation in the results was effectively avoided; and surface runoff simulation exhibits relatively high precision.