Abstract: A series of channel experiments were carried out to investigate salt intrusion lengths with different tide ranges. The experiments show that there is a critical tide range value, with which the salt intrusion length reaches its minimum. Tide range increasing results in rapidly decreasing salt intrusion length if tide range is smaller than the critical value, while salt intrusion length slowly increases with increasing tide range if tide range is higher than critical value. Analyses of fresh and salt water mixing based on the experiments data uncovered the essence of the phenomena: ① The stratification is broken during the tide range changing, which changes the forces resulting in salt intrusion; ② Gain of salt is mainly due to the exchange flow if fresh water and salt water is stratified, stronger tide strength could damp the stratification and reduce the exchange flow salt flux, causes smaller salt intrusion length; ③ While if fresh water and salt water is mixing well, gain of salt is mainly due to diffusion, stronger tide strength could provide stronger diffusion, causes longer salt intrusion length.