Abstract: The bulb turbine has become a good model for developing low head or ultra-low head water resources and tidal energy, but the problems of vibration and insufficient output frequently occur. Certain differences exist between the performance of its prototype and that predicted by the model test, due to the scaling effects and the small ratio of water head to its runner diameter (H/D1). This paper presents numerical simulations of a prototype bulb turbine working under two conditions of considering and neglecting the effects of free surface and water gravity, focusing on analysis of the effects in terms of its running performance and the cause of its insufficient output through examining the details of its internal flows. The results show that under the effects of free surface and water gravity, the turbine features a poor uniformity of the flow around its inlet and a gradual pressure increasing from top to bottom, and consequently a blade undergoes periodic pressure fluctuations during its rotation. And the blades at different positions contribute different torques to the shaft and the power output and efficiency of the bulb turbine are decreased considerably.