Abstract: This paper took bulb tubular turbine of hydropower station in Changsha multi-purpose Project as the research object to build a geometrical model, and numerical simulation of whole flow passage was carried out using SST k-ω turbulence model. By comparing the results of with gravity field and without gravity field, we analyzed the influence of gravity field on the performance prediction of bulb tubular turbine. The results show that the gravity field can be ignored when calculating external characteristics of bulb tubular turbine such as efficiency, output, but it should be considered when predicting cavitation performance and calculating structure characteristics.