The invention discloses a method for measuring elevations to evaluate the central state of a shaft system of a half-speed unit. The method comprises the steps of 1, determining a plurality of first measuring points on a spring base, determining two second measuring points on each bearing box, and determining two third measuring points on each rotor; 2, measuring under a hot state or cold state ofthe half-speed unit to obtain the elevations of the first measuring points and the second measuring points; 3, measuring under the cold state of the half-speed unit to obtain the elevations of the third measuring points; and 4, generating a curve in a coordinate axis according to the obtained elevation data, and comparing the curve with a standard curve to determine the central state of the shaftsystem of the half-speed unit. By means of the method, the elevations of the measuring points are measured under the condition that a backing wheel does not need to be disassembled according to the determined measuring points, and the central state of the shaft system of the half-speed unit is evaluated according to the curve generated by the elevations, so that the central deviation of the backing wheel of the rotor shaft system is calculated, whether the unit can continue to operate safely or not is evaluated, unnecessary work is avoided, a large amount of manpower is saved, and overhaul period is shortened.