In order to define the relationship between water age and hydraulic retention time of Taihu Lake, and the weights of the main influencing factors. This research combined the concepts about water age and hydraulic retention time, coupling convection diffusion and first-order degradation equation, An improved mathematical model of lake water age is established. Moreover, the parameters of the model are calibrated and verified. The sensitivity analysis of lake volume, wind direction, wind speed and initial water level was carried out with the method of standardized multiple stepwise regression. The results showed that: The concept of water age can be considered as the time required for 100% degradation of material concentration under ideal circumstances, which has a proportional relationship of 63% with hydraulic retention time. The weight of the four influencing factors are 64.41%, 8.56%, 9.57% and 17.46%, respectively, that is, the amount of water entering and leaving the lake and the initial water level can explain more than 80% of the phenomenon, which is the decisive factor affecting the water age.