Abstract: By solving Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulence model,a three-dimensional viscous steady numerical simulation was performed for investigating the vortex development and loss mechanism in a 1.5-stage high load test turbine designed by a controlled vortex.Numerical results show that the suction side leg of horseshoe vortex does not vanish immediately after reaching the suction side in high load rotor cascade,then overlaps with the pressure side leg of horseshoe vortex and finally enters into the flow passage.The suction side leg of horseshoe vortex is right under the pressure side leg of horseshoe vortex and adheres to the suction side of the rotor near the end-wall,but doesn’t intertwine with each other throughout the downstream development process.Suffering from the high transverse pressure gradient in the rotor passage,the end-wall boundary layer is pushed towards the suction side directly,which causes the inadequate development of the passage vortex.The strong whirligig does not happen in the passage vortex,and the vortex appears in the form of sheets.The low-energy fluid keeps moving towards the corner of the suction side and expands towards mid-span,then mixes with the main flow which causes the loss increment in rotor.Thus,the key to improve the high load turbine stage efficiency is the improvement of the rotor performance.