Disclosed is a draft tube for a water turbine. A rectification cylinder which has openings at two ends, i.e. the front end and the rear end, is provided in a passage of the draft tube. The rectification cylinder is coaxial with a conical tube segment. A water flow of the water turbine flows out from a runner outlet and enters the conical tube segment of the draft tube. After the water flow enters the conical tube segment, the water pressure in the conical tube segment has the characteristic of being low in the axis but high on the perimeter. After the water flow meets the rectification cylinder which has openings at the front end and the rear end in some position of the conical tube segment, the water pressure on the axis may be increased, but the water pressure on the perimeter may be reduced, so that the difference of pressure between the axis and the perimeter may be reduced, and therefore the suppression of the generation and influence of an eccentric vortex strip can be achieved. The draft tube can effectively suppress or reduce backflow phenomena without influencing an output power of the water turbine, thereby making the water flow more stable and extending the time during which the water turbine continuously does work by the propelling of the water flow. On the premise of not increasing the height of a straight conical tube, i.e. not increasing the lower excavation of a hydropower station, the objective of increasing an energy recovery coefficient is achieved, so that the energy conversion efficiency of the water turbine is improved.