Abstract: The waterlogging control methodology and the drainage capacity are deeply influenced by the flood level of outer river which encounters interzone rainstorm,so it's necessary to study the risk rule of the interzone rainstorm encountering with the outside river flood.The Copula theory is applied to construct the joint distribution of interne rainstorm and the flood level of the outer river.The density of the bivariate joint probability is used to describe the chance of the bivariate encounter.The methods used to analyae the risk probability and the risk return period is presented.The case study is conducted and the resups indicate that the Cbpula function is able to simulate well the joint distribution of the interzone rainstorm and the flood level of outer Lianjiang river in Yangshan of Guangdong Province.The bivariate joint probability density clearly shows the characteristic of the positive skew distribution.The interzone rainstorm with a return period of no more than ten-year is most likely to encounter the flood level of outer river with the same frequencies,while for the interzone rainstorm with a return period of more than ten-year,the return period of the flood level of outer river corresponding to the maximal chance of encountering is smaller than that of the interzone rainstorm.For any return period of drainage,there are many kinds of combined schemes between the return period of the interzone rainstorm and the flood level of the outer river which are of the inverse correlation,and the return period of the interzone rainstorm is bigger that of drainage in any combined scheme.