With the increase in the use of air conditioning in China, the sustainable development of building air conditioning systems has become an important part of achieving the national ‘dual carbon’ goal. As the core equipment in the central air-conditioning system, chiller plant has a significant impact on building energy consumption and carbon emissions. The on-site testing technology of the actual coefficient of performance (COP_d) of chiller units is focused on, and the testing practice of the coefficient of performance (COP) of chiller units is analysed in Ningbo area as an example. Through the calculation method of JGJ/T 177—2009 ‘Energy Efficiency Testing Standard for Public Buildings’, based on the ratio of cooling supply to input power, combined with measurement tools such as ultrasonic flow meter, thermometer and multi-functional power meter, the on-site COP of chiller units was determined, and some precautions for the testing process were proposed. It is found that the COP_d of most chiller units in Ningbo is low, and energy-saving modifications are needed to improve energy efficiency. Suggestions are also made to strengthen routine maintenance, scientific management, trade-in strategy and government support in order to promote building energy efficiency retrofit and help green, low-carbon and sustainable development.