Since January 2019, the dilution steam generation of the 330 kt/a ethylene plant in Daqing has gradually decreased, resulting in the increased flow of outsourcing medium pressure steam. By comprehensively investigating the dilution steam system, it was confirmed that the decrease of dilution steam generation was caused by the reduced heat exchanger capacity of dilution steam system and the leakage in the liquid level control valve of furnace water recovery tank. By carrying out high pressure water cleaning of the heat exchanger and replacing the leaked valve, the dilution steam generation increased, and energy saving and emission reduction were achieved.