The method for retrofitting a double flow steam turbine includes opening an outer casing of the steam turbine, removing the blade carriers and the stator blades, removing the inner casing with inlet belt, removing the double flow rotor and the rotor blades. Thus, the method includes the further steps of providing a new single flow rotor and new rotor blades different from the rotor blades connected to the double flow rotor, providing balancers connected to the single flow rotor, to balance the axial thrust, providing new stator blade carriers and new stator blades different from the stator blades associated to the double flow rotor, providing a new inner casing with inlet belt, providing a passage between the inner casing and the outer casing, closing the outer casing.