Abstract: As the convertible energy of a hydroelectric generating set depends largely on the potential energy of its head water relative to tail water, reservoir stage and tail water level are major factors influencing its output capacity and running stability, especially for a tubular turbine that, working in the condition of relatively small potential energy differences, is more sensitive to variation in power output and stability of the water levels. In this work, we simulated and analysed the flows in a tubular turbine working by the design operation curve. Results show that the reservoir stage and tail water level are major factors that affect its power output at fixed working conditions, and that by adjusting them the output can be improved considerably. In addition, by adjusting both the flow structures inside the turbine and these water levels, the stability of the turbine set can be enhanced. The conclusion has been verified using in-situ measurements.