The drastic speed fluctuation of ammonia compressor driven by a steam turbine in a certain project results in the fluctuation of high and medium steam pressure and the deterioration of speed regulation, which leads to the runaway speed of the steam turbine and triggers the over-speed interlock.The over-speed of the steam turbine exceeds 14 000 r/min due to the failure of quick-closing valve.By analyzing the causes, it can be seen that the pilot oil valve particles can only cause the reduction of the speed regulating valveopening instead of the speed fluctuation of the steam turbine.Therefore, the influence of high and medium pressure steam pipe network pressure, load fluctuation of the compressor and pilot oil valve particles on the speed of the steam turbine is excluded.The on-site calibration records of the speed control system are analyzed to confirm that the abnormal operation of the electro-hydraulic converter results in the accident.The test and disassembly of the electro-hydraulic converter show that the blockage of filter screen caused by lubricating oil sludge inside the converter is the fundamental reason for the abnormal operation of the electro-hydraulic converter.At the same time, it is confirmed that the dead-point position of the quick-closing valve causes the serious overspeed resulted from the failure of the quick-closing valve.Adjusting the P\I value of the speed regulating system and tracking the actual speed with the target speed can reduce the frequency and amplitude of the speed fluctuation of the steam turbine.The replacement of the electro-hydraulic converter and the static debugging of the speed control system can eliminate the faults, and a duplex strainer is provided before the electro-hydraulic converter to improve the stability of the speed control system.