This article is in order to solve the existing problems in the central air-conditioning system, such as energy efficiency information scattered and hard to be shared, designs a central air-conditioning energy efficiency monitoring system. According to the existing energy efficiency monitoring means and efficiency calculation methods, builds an energy efficiency evaluation system which is combined with the central air-conditioning diagnosis ontology library, to display energy efficiency monitoring results and diagnostic decisions for the central air-conditioning system devices and subsystems. This platform can provide convenience for energy efficiency monitoring of the central air-conditioning, it can also provide conditions for further maintenance for energy saving.