Abstract: A gas-liquid two-phase flow state can easily cause abnormal pressure fluctuation in the long-distance water conveyance pipeline, particularly when closing the valves and leaks. The water hammer can be posed a greatly destructive impact on the safe operation of the pipeline. Therefore, it is necessary to determine the abnormal water pressure in the complex water transmission pipeline, particularly for the maintenance cost of the equipment or the possible adverse impact of the failure. In this study, a gas transient flow model was established to consider the instantaneous acceleration-based (IAB) unsteady friction using the Method of Characteristic (MOC) and Discrete Gaseous Cavity Model (DGCM). In this study, pipe models of reservoir, leak point, air tank, valve and reservoir were selected to simulate the effects of air tank, initial flow rate, initial void fraction, leak position and leakage rate on valve end pressure. The results showed that the air tank was significantly reduced the peak value of transient pressure in the water hammer, leading to attenuating the pressure rise of the pipeline. The duration was shorter from the pressure decay to the stability, indicating the better protection of the water hammer. Specifically, the pressure peak value of the water hammer dropped sharply from 29.69 to 8.38 m after installing the air tank. The installation position of the air tank was also a sensitive factor. Once the installation position of the air tank was closer to the downstream, there was a smaller peak value of transient pressure in the water hammer at the valve end, the shorter time when the pressure reached the peak value, the greater the attenuation rate, and the shorter pressure fluctuation time, indicating the better protection against the water hammer. The void fraction and initial flow rate posed a great influence on the peak value of transient pressure in the water hammer at the end of the valve. When the initial flow increased from 0.005 to 0.04 m3/s, the peak pressure was also surged from 6.56 to 17.47 m, indicating the longer period of pressure vibration and the stable time. When the void fraction increased from 0.000 1 to 0.01, the peak pressure decreased from 11.50 to 8.37 m. The larger initial flow rate or the higher void fraction was greatly contributed to the lower peak value of pressure, and the shorter time for the pressure in the pipe to reach stability. In addition, there was no change in the peak pressure of the pipeline, when the leakage of the pipeline was less than 1% of the initial flow. It infers that the small leakage had little impact on the pipeline pressure. However, the peak pressure in the pipe decreased sharply, when the leakage was more than 1% of the initial flow. There were all the same with the pressure peaks at different leakage positions, when the leakage position was the upstream of the air tank. But when the leakage position was the downstream of the air tank, the peak pressure decreased significantly. Consequently, the gas-liquid two-phase flow model can be widely expected to predict the gas-liquid two-phase transient flow, indicating an excellent application for the water hammer protection in the complex water transmission pipeline. In terms of water hammer protection measures, an air tank can be installed in the water transmission system simply and effectively. If the leak location was to be predicted, further simulation can be slightly changed in the pressure peaks during leakage.