The effects of direct extracts of compost (DEC),aerated fermentation extracts of compost (AFEC) and non-aerated fermentation extracts of compost (NAFEC) on cucumber growth and the action mechanisms were evaluated based on the structure and activity analysis of humic-like substances.AFEC increased cucumber growth most significantly,followed by DEC and NAFEC,which was insignificant compared to the control treatment.Humic-like substances from compost extracts played an important role in promoting cucumber growth.Application of humic-like substances stimulated auxin-like activity and increased chlorophyll content and nitrogen accumulation in plants.The positive auxin-like activity of humic-like substances could be attributed to the relative distribution of special carbon groups,such as those with a large amount of peptidic and carbohydratic groups or with a low content of phenolic groups.In conclusion,the best growth promotion by application of AFEC was mainly attributed to the humic-like substances in the AFEC.