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The performance of draft tube plays an important role in the efficiency and stability of hydraulic turbine. To improve the hydraulic performance of an axial flow fixed-blade hydro-turbine, the depth of draft tube was studied by changing the draft tube cone height using numerical simulation. Three different modified cases of draft tube depth were conducted and analyzed. The results showed that the recovery coefficients and the diffusion losses of draft tube were increased with its depth increasing within limits. However, the fraction losses were decreased with it. Thus, the draft tube depth of 3.0 D 1 was chosen as the initial scheme by comprehensive consideration of the efficiency and economic factors. Besides, the turbulence kinetic energy, pressure fluctuations and force components acting on the runner were compared to judge the effect of draft tube depth on stability. The results showed that the increase of depth was favorable for the flow field stability at rated conditions. The turbulence kinetic energy at both the inlet and outlet of the cone was reduced with the heightening of draft tube cone, which also proved the flow field interaction between the draft tube and runner. The flow improvement in draft tube would in turn reduce the disturbance to runner outlet, which also explained the decreases of the peak to peak value and force components in runner. The areas lower than the critical cavitation pressure on the suction side of blades was almost unchanged compared with the original draft tube.