Based on the characteristics of the better correlativity between annual runoff process and annual maximum peak flow series, the paper studied flood law in microcosmic and macroscopic aspects by applying the analysis method of power spectrum and grey relation. The results of power spectrum analysis show that the annual maximal peak flow series at upper reaches of the Yellow River has a few period harmonics, and they can not pass test of the building-up determinacy model. So it is difficult to forecast the maximum peak flow by establishing determination model. Through the correlativity statistic analysis and grey relational degree computation between macula cyclic change and flood of upper reaches of the Yellow River ,the results show that the big dimension weather system of upper reaches of the Yellow River and macula have very good correlativity. Therefore, it is possible to forecast the maximum peak flow according to the change of macula.