The diurnal responses of photosynthetic rates (Pn), stomatal conductance (Gs), transpiration rates (Tr) and water use efficiency (WUE) to the daily march of microclimatic conditions were simulated by the coupled model of apple leaves gas exchange. Values of Pn and Gs increased in the morning, reached a maximum about 8:00 AM, decreased to lower values in the midday, recovered to the second maximum about 4:00 PM, and decreased in late afternoon, creating an asymmetrical double-peak curve on sunny days. The diurnal change of Tr was similar to that of Pn and Gs, following “n” shape curves on clear days, reached the maximum after midday. But the midday depressions of Pn and Gs were more strongly expressed than Tr. On clear days, the peak of WUE was reached early after sunrise; it then decreased to a stable level maintained through most of the day until falling to zero at sunset. The mathematical simulation showed the diurnal course of gas exchange was driven mainly by radiation and midday depressions mostly caused by the reduction of leaf water potential and relative humidity. These established models were tested by the observation data in an apple (Malus pumila Mill.‘Fuji’) orchard (latitude 40°13′ north, longitude 116°13′ east, altitude 79 m). There was a satisfactory correspondence between measured and simulated values of Pn in different photosynthetically active radiation (PAR), air relative humidity (RH), air temperature (Ta), air CO2 concentration. The diurnal variations of Pn and Gs in upper layers of canopy, compared with the model predictions, showed a good general agreement. Only a slight overestimation of Tr was in our study, which might be caused by leaves shading and non-linear relationship between Pn and PAR.