Promoting the use of smaller diameter tube in room air conditioner is beneficial to reduce copper consumption and refrigerant charge, but may cause reduction of air conditioner performance, so a design method is needed. This paper presents a simulation-based design method for air conditioner with smaller diameter tube. The new method combines heat exchanger simulator and knowledge-based evolution method optimizer for designing air conditioner heat exchanger with smaller diameter tube. The simulation-based design method is illustrated in detail by an air conditioner of replacing 7mm tube indoor unit heat exchanger and 9.52mm tube outdoor unit heat exchanger with 5mm tube. Case study shows that the cost of the designed air conditioner with 5mm copper tube is 17.3% lower than that of the original one while the performance deviation between these two air conditioners is less than 0.7%.