For the most common fresh air handling way supplying fresh air directly into the rooms after it has been handled in fan coil with outdoor air systems, the paper gives the design method of i d pattern in winter air conditioning operating mode, which determines the state of supply air of fan coil units and the final parameter of winter fresh air througn the ratio of fresh air. For selecting the final parameter of fresh air after it has been handled,the authors gives three conditions:Winter fresh air is handled to room enthalpy;Winter fresh air is handled between room enthalpy line and room temperature line; Winter fresh air is handled to room temperature or a little lower than room temperature. The comparing result show that the second method is desirable after comparison.