The invention relates to a pretreatment method for a cobalt-based catalyst for preparing oil through Fischer-Tropsch synthesis. The pretreatment method comprises the five steps of catalyst roasting, catalyst reduction, synthetic gas treatment, CO treatment and H2 re-reduction, and is suitable for a Fischer-Tropsch synthesis oil-preparation reaction of carbon materials, such as activated carbon, mesoporous carbon and carbon honeycomb monazite, and a cobalt-based catalyst system loaded with carriers such as silica gel and aluminum oxide in a gas-liquid-solid three-phase slurry bed reactor and agas-solid two-phase fixed bed reactor. By adopting the pretreatment process, the activity of the cobalt-based catalyst can be effectively improved, methane selectivity is reduced, and C<5+> selectivity is improved.