Abstract: In order to understand the effects of shading on the growth of Alnus formosana,100%,50% and 15% of transmittance were simulated according to the light regimes in the forest opening,forest gap and forest canopy. Studies were made of the growth and physiological characteristics of A. formosana seedlings planted in different shading treatments. The results showed that with shading intensity increasing,plant height and base diameter showed the trend of first increasing and then decreasing,while net photosynthetic rate(Pn),transpiration rate(Tr) and stomatal conductance(Gs) gradually reduced except for intercellular CO₂ concentration(Ci). Under different shading conditions,chlorophyll a(Chla),chlorophyll b(Chlb) and total chlorophyll content increased with transmittance decreasing,however,carotenoids(Car) content,chlorophyll a/b(Chla/b) and lamina mass per unit area(LMA) were opposite. In addition,with the reduction of light intensity,soluble sugar exhibited first decreasing and then increasing,while proline(pro) content gradually increased in leaves of A. formosana seedlings. Therefore, it was suggested that A. formosana seedlings could not adapt to the high-light environments outside forest,but moderate shading,that is to say,canopy gaps would benefit the seedling growth. At the same time,it also provided a scientific reference for forestation,breeding and vegetation recovery.