【Objective】Neonicotinoid pesticides are one of the most commonly used insecticides in citrus orchards in the hilly areas of southern China. The long-term fertilization of orchards can alter the content and structure of soil organic matter, which in turn affects the adsorption behavior of pesticides in the soil. Therefore, gaining an in-depth understanding of the adsorption behavior of a typical neonicotinoid pesticide (imidacloprid) on soil humus in orchards is crucial for improving our knowledge of the environmental behavior of this class of pesticides in soil. This will provide a theoretical basis for preventing and controlling pesticide surface pollution at its source.【Method】Humic acid (HA) fractions (F1 to F10), low carbon humin (HuL) and high carbon humin (HuH) were extracted from soils (0~20 cm) with different cultivation chronosequences (10, 30 and 50 years). Adsorption isotherm experiments of imidacloprid on these humic fractions were conducted. 【Result】The adsorption isotherms of imidacloprid by humic acid (HA) fractions (F1~F10), and HuL and HuH fractions all fit the Freundlich model well, with R2 greater than 0.9. The adsorption affinity of humic acid for imidacloprid is about 100 times higher than that of HuL and HuH. The Kd values of imidacloprid ranged from 523.1 to 5 276L?kg-1, 543.3 to 5 717 L?kg-1, and 520.2 to 5 980 L?kg-1 at Ce of 0.5, 2.0, and 3.0 mg?L-1, respectively. All of them increased slightly with the increase of planting years. The KOC values of imidacloprid were positively correlated with aromatic C and negatively correlated with alkyl C of HA fractions (F1~F10). Also, the carbonyl C and carboxyl C of HuL were the key carbon functional groups controlling imidacloprid adsorption, while the alkyl C of HuH was the key carbon functional group controlling imidacloprid adsorption. 【Conclusion】Therefore, long-term cultivation can improve the adsorption of imidacloprid by humic substances to a certain extent, and the aromatic carbon structure of humic acid is the key structure regulating the adsorption of imidacloprid while the aliphatic carbon structure of humin is the key structure regulating the adsorption of imidacloprid.