The effects of different forest gap sizes on the regeneration pattern of the undergrowth of secondary Poplar-birch were studied,which provided theoretical basis and reference for the artificial near-natural simulation of forest gap disturbance renewal.Taking the secondary Poplar-birch forest in the Southern Greater Xing'an Mountains,Inner Mongolia as the research object,the 6hm2 large plots were divided into the upper part,the middle and the lower part of slopes according to the altitude gradient.Each slope was divided into 4 forest gap area grades,and each grade was selected 3 forest gaps of different sizes,as three repetitions,36 forest gaps are selected for each slope position,and 5regeneration seedling survey samples are set in each forest gap.At the same time,36 control samples (CK) of the same area were set.A total of 180 regeneration seedlings samples and 180 CK samples were set up and surveyed in the forest gaps.The results showed that the forest gap had the tendency to decrease the regeneration density of the original pioneer species (Mongolian scorpion); the sorghum in the forest gap was the main renewed species.From the lower part,to the middle part and the upper part of slopes,the regeneration seedling density of Quercus mongolica accounted for 61.9%,50% and 39.86% of the total renewal density respectively.The regenerated seedlings in the forest gaps are mainly low-lying plants (<60 cm),and the number of regenerated seedlings show a decrease with an increase of height level in different slope positions and different forest gap areas.The number of regenerated seedlings in different heights of different slopes showed that there were more large forest gaps than small ones.The slope position and forest gap area all had significant effects on the algae diameter of Quercus mongolica (p<0.05),forest gaps could significantly promote the regeneration of Quercus mongolica.The growth of seedlings was the largest in the forest gap area of 80-100 m ².The diameter of the seedlings in the forest gapin the lower part and the upper part of slopes were higher than those in the forest,and there was no such performance in the middle part of slopes.In general,forest gaps can significantly promote the regeneration and growth of undergrowth species,and have a tendency to reduce the density of regeneration of the original pioneer speciesMongolian mongolica.