On the account of topography and strata of mountains and valleys in Southwest China, the temperature and salinity of regional groundwater system are different from those of local flow system. Induced by climate change or human behaviors, surface water level rises and even inundates the discharge point of groundwater, exerting impact on the mix between groundwater and surface water in the regional discharging area. In the present paper, variations in the features of the mix between salt water and fresh water flows are researched via sand box physical model and SEAWAT simulation from the aspect of salinity distribution. The results are concluded as follows: under the actions of both hydraulic head difference and density difference between regional groundwater and surface water, a mixed transition zone (from low salinity water to high salinity water) with parabolic boundary is generated by hydrodynamic dispersion by the intrusion of fresh water into aquifer. Furthermore, simulations of the mix between salt water and saline water with density difference demonstrate that when the density difference gets larger, the backward distance of the mixed transition zone is longer, and the boundary is more prolate. The research results lay a foundation for deep research on the mix characteristics between different levels of groundwater systems in Southwest China.