This invention involves a method of stepwise oil-water separation by oil-gas washing in Fischer-Tropsch process. At first, after mixed washing and hydrocarbon composition, light oil containing about 0.5-6% synthetic water and low gas containing trace hydrocarbon enters from the left-side end socket into the primary oil-water separator, and sewage containing 0.05-0.5% oil enters into the separator from the right-side end socket; purified low gas is discharged from the top and the mixed liquid enters into in-depth dehydrator after being discharged. Discharged light oil contains about 0.05-0.5% of water; sewage on the left side is discharged after entering water drum and sewage after deoiling on the right side is discharged on the right side of clapboard and enters into in-depth oil eliminator. Discharged oil contains about 0.03-0.6% water; after in-depth deoiling and dehydration through conjugate fiber module weaved in particular form in in-depth dehydrator and oil eliminator, light oil containing less than 0.02% water and sewage containing less than 0.01% oil are discharged separately. The method of combined oil-water separation and stepwise separation provided by this invention is able to make fast and efficient oil dehydration and sewage deoiling, and to recycle oil to the maximum extent.