Elucidating the effect of forest gap on early seedlings growth and functional traits among different light-demanding tree species is of great significance to reveal the underlying mechanism by which seedling regeneration is subject to microenvironment of gap in secondary forest. In this study, the seedlings of tree species, including shade-tolerant evergreen (C. camphora) and light-demanding deciduous (L. formosana) species in subtropical secondary forest, were used to study the effect of gap size (large gap, D/H=1.5-2.0; medium gap, D/H=1.0-1.5; small gap, D/H=0.5-1.0) on growth and functional traits at the early seedling stage (1-3 years old). The research results showed that (1) the significant growth of seedlings was observed among gap sizes. The growth of 2, 3-year-old C. camphora seedlings and 1-3-year-old L. formosana seedlings were significantly enhanced when growing in medium gap and big gap, respectively. (2) The relationships between environmental factors of forest gap and functional traits of seedlings were examined by using redundancy analysis. We found that the functional traits of C. camphora seedlings were correlated strongly with soil organic matter, hydrolyzable nitrogen, pH value and available phosphorus in forest gap, whereas the functional traits of L. formosana seedlings were mainly affected by pH value, soil organic matter, hydrolyzable nitrogen, soil water content, light intensity of canopy and available phosphorus. (3) Maintaining the higher root mass ratio, specific root length, leaf carbon to nitrogen ratio and leaf carbon to phosphorus ratio were key ecophysiological traits to reduce the negative impact of environmental change on the seedlings growth in forest gap.