One of the most challenging technical barriers for fixed bed Fischer-Tropsch (FT) reactors is the formation of a hot spot in the catalyst bed because of the tremendous release of reaction heat. Increasing the thermal conductivity of the catalyst bed is a direct and efficient solution to solve this problem. In this work, Computational Fluid Dynamics (CFD) simulations were carried out to study the effect of the thermal conductivity of a catalyst bed on the Fischer-Tropsch synthesis (FTS) fixed bed reactor. A reasonable constraint was defined for the temperature rise in the catalyst bed, in order to ensure system safety and stability of the catalyst for long-term operation. The catalyst activity and space velocity (SV) were modified in each simulation experiment to ensure the maximum temperature in the catalyst bed did not exceed the temperature rise constraint. The potential maximum productivity of the specific catalyst bed was then predicted and the optimized catalyst activity and SV were obtained, which provided valuable guidance for practical FTS applications. The results showed that by applying a catalyst bed with higher thermal conductivity, better temperature control of the system was achieved, and a higher FTS reaction rate in the catalyst was ensured, which resulted in higher heavier hydrocarbon productivity.