Aims Differences of photosynthesis and chlorophyll fluorescence characteristics between flat young leaves (young leaves) and fully grown leaves (mature leaves) of cotton were compared to study the adaptation mechanisms of photoinhibition and photoprotection on young leaves under strong sunlight. Methods We measured gas-exchange and chlorophyll fluorescence of young and mature leaves of cotton under field conditions and obtained rapid light curves with a Dual-PAM100 using an internal program. Important findings There were significant differences between young and mature leaves in photosynthesis and fluorescence characteristics. Young leaves had lower chlorophyll content (Chl) and stomatal conductance (Gs), which accounted for lower net photosynthetic rate (Pn), which in turn resulted in lower actual photochemical efficiency (ΦPSII) and photochemical quenching (qP) in photosystem II (PSII). Below 1 800 μmol·m–2·s–1, the higher cyclic electron flow (CEF) that was useful for composing ATP is one of the reasons for higher photosynthetic capacity in mature leaves. Lower light saturation point (LSP) was more susceptible to photoinhibition in young leaves under the same light; however, the magnitude of diurnal variation of its maximum photochemical efficiency of PSII (Fv/Fm) was significantly less than that of mature leaves. We suggest that photoprotective mechanisms, including direct quenching of reactive oxygen species by abundant carotenoids, photorespiration (Pr), non-photochemical quenching (NPQ) and the cyclic electron flow around PSI (PSI-CEF), can effectively dissipate the excess energy in young leaves to avoid photoinhibition under high irradiance.