Abstract: In order to further understand the leading influential driving forces and their cooperative mechanism in saltwater intrusion of riverine estuaries in dry season, 3-D numerical simulation experiments were conducted in this study with a simplified estuary which was forced by sinusoidal tides of constant tidal ranges and main tidal constituents respectively at different runoff flow. The results indicate that in tidal saline equilibrium, with variation of relative strength of the runoff and tide, the shape and the location of salt water intrusion are automatically adjusted and end with balanced salt transportation. When the runoff rates are less than 3 000 m³/s and tidal ranges less than 2 m, there are obvious differences in the variability of maximum intrusion lengths with tides under different runoff flow. For weak mixing estuary, during the period time of spring and neap, the saline water presents fortnight non-equilibrium and their data points of tidal maximum intrusion length with respect to tidal range change clockwise like a loop around that of the equilibrium state. The density circulation induced by runoff and the tidal mixing collaborate and develop with interaction. The contrast of their intensity dominates the significant distinction of saline intrusion length in riverine estuaries.