Abstract: To reveal the evolutional characteristics of a stream-groundwater system, this paper takes the seasonal losing stream-groundwater system with stratified sediments as the research object. The criterion for the hydraulic connected states and the mechanism of the seepage in the system are analyzed. The variation types and mechanism of hydraulic connected states are clarified. A theoretical framework of stream-groundwater system hydraulic connected states is developed, and six stream-groundwater system models with different stratified sediment types are constructed. The evolution and variation of hydraulic connected states of the six stream-groundwater models are simulated by numerical simulations, and the results show that the wetting front curve and saturation front curve are of great importance to describe the evolutional process of the hydraulic connected states and can be regarded as characteristic curves for analysis. The difference of permeability and capillary force caused by different types of sediments is the fundamental driving force for the variation of the evolutional process. The variation types of wetting front curve include steep, gentle slope or alternate between steep and gentle slopes, and the variation types of saturation front curve include backward, forward, or alternate between backward and forward. The results of this study provide theoretical support for further research on water quantity assessment, mass transport and energy transfer in stream-groundwater systems.