The effects of a rectifier on the hydraulic stability of the draft tube is explored with research based on a straight cone-shaped draft tube with constant sized diffusion, where the uninstalled rectifier and the installed rectifier in the straight cone section of the draft tube are simulated under different conditions of hydraulic stability. The internal pressure pulsating characteristics of the draft tube, the internal pressure and the output change of the runner, and the internal flow regime change of the draft tube are also analyzed. The results show that the pressure waveform of the draft tube is relatively stable when the rectifier is installed under the typical partial load condition, and the pressure fluctuation amplitude near the tube wall of draft tube is small, the peak value of the mixing peak is large and the frequency component is rich. The installation of a rectifier in the draft tube can guide and rectify the water flow, so that the flow regime of the rectifier inlet tends to be more stable, the differential pressure between the axis and the radial direction decreases, and the recirculation region and secondary flow inside the runner outlet and the straight cone section are reduced. In this way, the production of vortex belt can be inhibited, and the actual water flow and flow uniformity of the inlet cross section of turbine can be improved.