To explore the impact of surface roughness on the aerodynamic performance of a contra-rotating fan, the aerodynamic performance of a contra-rotating fan under different roughness combinations of shell, blade, and hub is investigated by numerical simulation. The study reveals the effects of different surface roughness levels on the fan’s aerodynamic performance and analyzes its flow field characteristics. The results show that as the roughness of the casing, blades, and hub increases, the efficiency of the contra-rotating fan decreases. The flow is more likely to experience boundary layer separation on rougher surfaces, disrupting smooth airflow along the blade surfaces and causing localized drag and mixing losses. The roughness of the blades and hub has a more significant impact on the fan’s total pressure efficiency than the casing roughness. It is recommended to choose the combination of casing, blade, and hub roughness of 6.2–20 mm as the casting process standard for a contra-rotating fan to obtain better aerodynamic performance.