Gas exchange, chlorophyll a fluorescence kinetics, chloroplast pigments and antioxidant enzymes were investigated to explore the development of photo protective mechanisms in soybean leaves from emergence to full expansion under field conditions. During leaf development, photosynthesis (Pn) gradually increased. Although the maximum quantum yield of PSII photochemistry (Fv/Fm) was quite high at the initial stages of leaf development, it was appreciably lower than that in fully expanded leaves. During daily courses, reversible decrease in Fv/Fm was noted at various stages, implying that no photodamage occurred. When exposed to irradiance, a substantial elevation in the actual PSII efficiency (φPSII) together with a remarkable decrease in non-photochemical quenching (NPQ) was observed with the process of leaf development. In this study, we observed that newly initiating leaves possessed relative larger xanthophyll pool size (V + A + Z)/Chl, but it began to decrease with leaf expanding until fully expanded. Further experiments revealed that the decline of relative xanthophyll cycle pool size during leaf expanding was owing to the fact that the chlorophyll contents per area increased faster than that of the xanthophyll cycle pigments. In addition, activities of the antioxidative enzymes, such as SOD, APX, CAT, POD and GR, were also enhanced at the early stages of leaf expansion. We suggested that photoprotective mechanisms, including xanthophyll cycle and antioxidant enzymic system, have developed firstly as soon as leaf emerged. It was the timely development of the mechanisms in young leaves that dissipate excess excitation energy protecting leaves against photodamage at the early stages of leaf development. The relative leaf water content increased with the process of leaf development and exhibited signicant negative linear correlation with NPQ, xanthophyll cycle pigments and antioxidant enzyme activities in soybean leaves. We proposed that the changes in leaf turgor might be involved in triggering the activation of photoprotective mechanisms together with high irradiance at the initiating stages of leaf expanding.