Based on a 13 L China Ⅵ heavyduty diesel engine, the influence of optimizing the accessories such as oil pump, water pump and air compressor on the fuel economy and emission characteristics of diesel engine was explored and analyzed through bench tests, and the contribution ratio of each accessory was acquired, which provided the direction for the subsequent optimization. The results show that replacing the lowflow oil pump, low-flow water pump and adopting the energysaving mode of air compressor or removing the air compressor on the original engine can reduce the specific fuel consumption to different degrees and improve the thermal efficiency both at the external characteristic working point and the optimal thermal efficiency working point. Under the two mentioned conditions, the optimization of oil pump resulted in an increase in exhaust temperature and NOx emission and a decrease in soot emission, while the optimization of water pump and air compressor led to a decrease in exhaust temperature and NOx emission and an increase in soot emission. At the optimal thermal efficiency working point, among the three optimization accessories, the optimization of water pump had the largest contribution to the improvement of thermal efficiency as much as 43.56%, while the optimization contributions of oil pump and air compressor were both about 28%.