Nowadays the safety of the air conditioning systems is paid more attention to. All cold outdoor air systems due to their particular advantages such as credible safety, better indoor air quality, saving more building space and being in favor of the energy conservation in some given conditions [1] are discussed in this paper. The composing of all cold outdoor air system is outlined and the mathematical models of energy consumption evaluation are presented. With an actual example by using the method of equivalent weight full load operation time, the influence factors to the annual primary energy consumptions in all cold outdoor air systems, such as: the dew point temperature, the COP (Coefficient of Performance) of chiller, the equivalent weight full load operation time of chiller and the efficiency of enthalpy exchanger etc. are compared and analyzed. And the optimal application modes for energy conservation of all cold outdoor air systems are proposed. These can be good references for the application of all cold outdoor air systems.© 2006 ASME