Stomata are the channels by which plants exchange water vapor and carbon dioxide with the environment. Clarifying the change from stomatal limitations (SL) to non-stomatal limitations (NSL) of photosynthesis and their critical conditions is vital for accurately recognizing the degree of crop drought and formulating countermeasures. A field experiment was carried out from 2013 to 2015 to study the critical water conditions when maize photosynthesis changed from being limited by SL to NSL in different leaf positions under different degrees of water stress in different growth stages (3rd leaf stage, 7th leaf stage and jointing stage). Our results indicated that photosynthesis of maize leaves at different positions changed from being determined by SL to NSL under different water stress levels at different growth stages; moreover, maize photosynthesis changed from being directed by SL to NSL in the first fully expanded leaf at the top before the changes occurred in the third leaf. The effect of water stress during different growth stages on the maize leaf water content (LWC) at which photosynthesis changed from being limited by SL to NSL was not distinct. The changing point of SL at different leaf positions was closely related to the LWC, and the LWC at the changing point of SL was different at different leaf positions, which indicated that the change in SL is mainly determined by the leaf position and LWC, and its occurrence showed a decreasing trend from plant top to bottom. The LWC at which the SL transformation point occurred in the first fully expanded leaf at the top (75.5 % ± 1.5 %–75.7 % ± 1.3 %) was higher than that at which the change occurred in the third leaf at the top (73.2 % ± 1.1 %–73.4 % ± 1.6 %). The phenomenon of photosynthesis changing from being limited by SL to NSL occurred first in the first fully expanded leaf at the top; additionally, the LWC of the first fully expanded leaf at the top was the best indicator in maize under water stress and could be used as the critical condition marking the transformation of maize damage from water stress to damage from plant physiological and ecological stress. These results could provide a basis for the identification of crop drought disasters and their classification and provide a methodological reference for the identification and monitoring of drought in other crops.