Abstract: Based on view of the probability theory, probability explanations of net tainfall process over watershed and discharge hydrograph of watershed outlet are given.On the basis of these, the convolution integral equation is derived by theory of distributed function of random variate in probability theory, and probability meaning of the instantaneous unit hydrograph (IUH) of watershed is again obtained under obviouser conditions.Equivalent relationship among first order moment about the origin of IUH of watershed, lag time of watershed and average concentration flow time of watershed is proved, and a computational method of average concentration flow time of watershed is given.Finally, two methods of determining the IUH of watershed are presented by means of probability theory.