Since big flood occurred frequently in the Pearl River Delta in recent years and the return period of flood has changed,this article uses the GEV distribution model to get the frequency analysis of extreme high water level values of 35 tide stations in the Pearl River delta for about 50 years. On this basis this article divides the time series of about 50 years into several small time periods and establishes the probability density function curve and function curve of relationship between water level and the flood return period in each time period by Monte-Carlo random simulation to study the changing rule of the return period of flood further and analyze the mechanism. The result shows that there is an obvious increasing trend in water level of flood return period in the central hinterland of the Pear River Delta, but a little change in the upstream and entrance areas. An important reason for this change is that large-scale sand-excavating activities in the upper region have a negative impact on the river and the surrounding environment.