Abstract: In order to predict the vortex rope during the design process for hydro turbine, the relationship between the runner outlet’s swirling flow of Francis turbine and the vortex rope inside the draft tube was analyzed. A 180-meter head Francis turbine is numerically simulated under three typical operating conditions by using the Reynolds-averaged Navier-Stokes (RANS) equations. The shape of vortex rope visualizations was compared with the results of the model test for verifying the correctness of simulation. Then, the swirl number, the velocity profile of runner outlet and the swirl number along the centerline of draft tube cone was analyzed to explain vortex rope formation under typical conditions. The results show that the swirl number at outlet of runner increase with the decrease of operating flow rate. When the swirl number at the outlet of the runner is greater than the critical value, the helical vortex rope, the low velocity zone and the backflow are formed in the draft tube. Meanwhile, when the swirl number is less than the critical value, there is no obvious vortex rope forming in the draft tube.