Abstract: The world’ s largest coal-based Fischer-Tropsch synthesis plant with a capacity of 4 million tons per year was industrially applied by Ningxia Coal Industry,China Energy Group in 2016. The medium temperature Fischer-Tro- psch synthesis technology from Synfuels China Technology Co. ,Ltd. was first in a large-scale application. The Fischer- Tropsch synthesis unit consists of eight slurry bed reactors with an annual output of 500 thousand tons of intermediate products,and is characterized by many series of synthesis reactors,a large scale and complex configuration. Many problems were exposed during the test run of the plant,such as a low separation efficiency of high temperature oil gas and an unreasonable design of intermediate product stripper. Most of the problems have been solved,and the safe and sta- ble operation of the plant has been realized through technical improvement and optimization in the past two years. The amount of catalyst particles in high-temperature oil-gas is significantly reduced by increasing the number of cyclones, reducing their size,and increasing the inlet oil-gas velocity. The blockage of heat-exchange separator is avoided,and the effective separation of heavy oil,light oil and synthetic water is realized. The optimization operation is carried out from two aspects. One is to keep control on the temperature of heavy oil in the heat exchanger separator. The other is to increase the temperature of heavy oil feed in the stripper. When the balanced partial pressure of water in the heat-ex- change separator is controlled at 0. 26-0. 32 MPa,the synthetic water will not condense and the light oil will not con- tain heavy oil. At the same time,the temperature of heavy oil at the bottom of heat-exchange separator is controlled at 128-135 ℃ . On the other hand,the temperature of heavy oil entering the stripper is not less than 170 ℃ ,and the heat rebalance of stripper has been realized. The temperature of bottom of the stripper keeps at above 200 ℃ ,and the safety of the stripper operation can be ensured. The outlet pressure of each Fischer-Tropsch synthesis reactor is controlled at 2. 3 MPa,and the intake volume of the first section of each cycle gas compressor is controlled. The intake volume of the second section of the compressor is increased to realize the operation regulation of the synthetic tail gas volume to the de-carbonization device. These means improve the matching between the Fischer-Tropsch synthesis reactors and the de-carbonization unit,and the operation stability of the system. The operation results of the plant show that the average selectivities to CH4 ,C+ and C+ are 2. 90% ,96. 15% and 92. 8% ,respectively,and the average syngas consumption per ton oil is 5686 standard m3. The by-product steam with 2. 8 MPa is 4. 53 tons per ton of the Fischer-Tropsch inter- mediate products,and the synthetic water is 1. 12 tons per ton of the Fischer-Tropsch intermediate. These results show that the design target has been achieved.