Split air conditioner (SAC) plays a critical role for temperature control of buildings. In recent years, the manufacturing process of SAC tends to consider energy-saving and environmental protection factors to reduce air pollution and alleviate energy crisis. However, the design of environmental-friendly SAC will increase the manufacturing cost, thereby increasing the burden on air-conditioning companies. Therefore, the eco-cost analysis of SAC is of great significance for air conditioner manufacturers. To this end, a new SAC eco-cost assessment method based on activity based costing method is innovatively proposed in this paper. In this method, a K-nearest neighbor algorithm is used to deal with the missing data in SAC samples. An environmental and non-environmental cost assessment approach is developed based on the activity-based costing method to estimate the eco-cost of SAC over its whole life cycle. In addition, the impact of energy efficiency deviation on SAC eco-cost is numerically analyzed. Finally, the proposed eco-cost assessment method is programmed on a Gabi software, and validated using real SAC data from a Chinese air-conditioning company. Simulation results show that the proposed method can provide an estimate of the eco-cost of SAC, thus helping air-conditioning companies to improve their competitiveness in the future where energy-saving and emission reduction are increasingly important.