Abstract: To explore the properties of daily flow variation in the Yangtze River and its prediction by using a new approach, the issue of chaotic properities of daily flow at Yichang station has been addressed systematically and fully.Based on the obtained results,some new models have also been developed.The findings from this comprehensive exploration are divided into several parts.This paper contains the first part,in which the stress is put on optimum determination of delay time.The determination of delay time is very important for chaotic analysis.Two traditional methods(autocorrelation and real vector) are applied to this determination.In addition,a new method,named as general correlation function method,has been put forward. The results show that the general correlation function method is a reasonable and operational method for determination of delay time.