Abstract: The investigation focused on the design characteristic of root blade profile in the large meridional expansion turbines.And,the effect of root blade profile optimized design on the flow separation at vane inlet and radial pressure distribution at vane outlet was also studied.With the commercial numerical simulation solver,NUMECA,optimized design of root blade profile was applied into a large meridional expansion turbine.Then full three-dimensional numerical simulations on the original and optimized profiles were carried out.The numerical results show that,wider profile and convergent-divergent root profile would adjust radial distribution of massflow at inlet,weaken or even eliminate incoming flow separation at the upper endwall,hence controlling the flow loss in shroud;meanwhile,such profiles could improve mach number distribution and radial distribution of pressure at outlet,weaken or even eliminate radial flow of S₁ section separation in the suction side at exit,thereby reducing flow loss in hub.