Rainfall with time and space distribution is regarded as muster of infinitely great “drop of rainfall” from the view of random theory. Infiltration, evapotranspiration and flow concentration for rainfall of a point located in the watershed can be handled by hydrodynamics. The collective expression of infinitely great “drop of rainfall” with space distribution, i. e. , formations of runoff yield and flow concentration of watershed can be handled by probability theory. The above-mentioned idea not only resolves the runoff yield of watershed affected by space variations of rainfall and underlying surface condition, but also reveals physical nature of watershed instantaneous unit hydrograph(IUH), i. e. , proportionality and superposition of watershed flow concentration. Finally, a method for determining space distribution of flow concentration velocity is put forward.