Abstract: After switching the heating mode of a steam turbine in a gas-steam combined cycle unit, there was a sudden increase in vibration of the low-pressure rotor of the steam turbine, prompting an emergency switch back to the previous mode. Through the analysis of changes of operation parameters before and after the incident, the preliminary determination of the incident cause was the change in steam admission parameters of the low-pressure cylinder after mode switching. To solve this problem, the set value for the exhaust steam pressure of the intermediate-pressure cylinder was adjusted, and another mode switching test was conducted. During this test, the changes in steam admission parameters of the low-pressure cylinder were reduced, and there was no significant change in vibration after mode switching. This case can serve as a reference for analysing other similar incidents.