The poisoning of commercial coprecipitated Fe-Cu-K catalyst by oxygen in the Fischer-Tropsch synthesis was investigated, since synthesis gas obtained by the gasification of coal contains O-2. The pretreated catalysts were oxidized in a oxygen-containing syngas. The effects of poisoning methods, reaction temperature, and oxygen fed to catalyst on the catalytic behavior of reduced Fe-Cu-K catalyst in a fixed bed were studied for synthesis tests with the ratio H-2/CO of 2.3 containing O-2 5.16x10(-3) under 2.62 MPa and 524.2 K. The relative activity of catalyst exceeded 1 for the first few hours of poisoning by oxygen both in prepoisoning and poisoning in situ process, then decreased linearly with the increasing of oxygen fed to catalyst, and finally increased to the relative activity of 0.6-0.7. The activity decreased less rapidly poisoning in situ than that of prepoisoning. The gaseous hydrocarbons average molecular weight increased greatly when poisoned by oxygen. The wax weight percentage in products and the ratio of olefin to paraffin decreased as poisoned by oxygen. Oxygen also affects the composition of oils and the contents of oxygenated compounds formed in water phase.