In order to investigate the operational performance of the high-pressure (HP) cylinder towards a 300 MW boiler unit, the three-dimensional calculation method was applied to simulate the aerodynamic flow characteristics of the last five stage cylinder blades, including limiting streamline, isentropic efficiency along the blade height, and relative work amounts. Simulation results show that with the back pressure controlled at 3.4 MPa and the inlet steam flow decreasing to a certain extent, the flow state of the last three stage blades begins to deteriorate. When the back pressure continues to drop, the flow state becomes unstable at the blade root and the blade tip on the suction surface of the rotor blade, while the flow state is stable on the pressure surface of the rotor blade and the surface of the stationary blade. If the flow rate decreases, the flow separation area on the suction surface of the moving blade decreases, and the flow performance can be improved. With the decrease in the boiler load, the total-total isentropic efficiency for the last three stage blades of the HP cylinder gradually decreases. In addition, the isentropic efficiency at the blade root and blade tip is lower than that for the blade body, which is closely related to the flow separation phenomenon of the last three stages of moving blades at the root and tip. In the working condition of the intermedium-pressure adjustment valve, since the temperature variation is within a limited range, operational safety can be guaranteed under the design blade frequency and strength. This paper provides the upper and lower limits of the exhaust gas pressure for the HP cylinder under the intermedium-pressure adjustment valve participation mode concerning various working conditions as well as corresponding constraints. This is conducive to guiding the design and safe operation of the intermedium-pressure adjustment valve. Through adjusting the intermedium-pressure adjustment valve, a specific supply capacity of the unit can also be satisfied at the lower load, which is beneficial to the deep peak regulation of the unit.