Abstract: The unstable flow caused by vortex in the draft tube has impact on the safe operation of hydraulic turbines. For the further study of the flow characteristics in the draft tube, this paper took the francis turbine of Futang Hydropower Station as the research object, and numerical simulation of the whole flow passage of francis turbine under different working conditions were carried out using a standard k-ε turbulence model to analyze flow characteristics in the draft tube under different heads. The results show that: the negative pressure areas were found at entrance of draft tube when the guide vane was fully open, the negative pressure areas expanded gradually with the increase of heads; pressure were distributed spirally under the maximum head, i.e., the cavitation vortex formed in the negative pressure areas, which may lead to relatively strong vibration of hydroelectric generating units.