Abstract: In order to quantitatively analyze the relationships between the residual head and the water delivery time of the mutual water filling and emptying double-line navigation lock, a mathematical model for the navigation lock's filling and emptying is developed on the basis of the unsteady flow energy equation and the continuity equation for the navigation lock's filling and emptying system. Taking Changzhou navigation locks No.3 & 4 as an example, the reliability of the mathematical model is verified by the prototype observation data. This research is focused on the hydraulics concerning the mutual water filling and emptying system of the double-line navigation lock and the relationships between the residual head, water delivery time and the rate of water saving are revealed. The research results indicate that with the decrease of the residual water head, the water filling time of the ship lock increases constantly. Meanwhile, as the closing time of the unicom valve is pretty long, at the end of the closing period, the water level of the filling water lock chamber is higher than that of the emptying water lock chamber, resulting in the phenomenon of back flow, and the water saving rate of the navigation lock increases to 0.5 and then to a lower value. Therefore, from the point of water saving, the residual head value of 4 m is the best. Considering the contradiction between the water saving rate and the water filling time, the weight problem of the water filling time and the water saving rate in the selection of the residual water head is put forward, which can provide a reference for the optimization of the residual head of the mutual water filling and emptying double-line navigation lock.