[Objective] Biochar is an effective soil carbon sequestrator. In this study, a one year field experiment was carried out to explore variation of content and chemical structure of the humic substances (HS) in salt stress fluvo-aquic soil under wheat-corn rotation.[Method] The experiment was designed to have four treatments, that is, CK (control, no fertilizer);T1 ((NH₄)₂HPO₄ at 750 kg·hm^-2);T2 (peanut shell derived biochar 15 t·hm^-2 plus (NH₄)₂·HPO₄ at 750 kg·hm^-2);T3 (wood derived biochar 15 t·hm^-2 plus (NH₄)₂HPO₄ at 750 kg·hm^-2).[Results] Results show that biochar amendment increased the content of HS in the soil. Compared to the wheat growing season, the corn growing season had humin (Hu) increased in content by 0.54 g·kg^-1 and 0.35 g·kg^-1 in Treatments T2 and T3, respectively, and absorption peaks of the humic acid (HA), fulvic acid (FA) and Hu increased, too, in relative intensity at the 2 920 + 2 850 cm^-1 band, while it had relative area of the absorption peaks of HA and FA at the 1 630 cm^-1 band decreased by 2.46% and 5.77% in Treatment T2. Compared to Treatment T3, Treatment T2 had relative area of the absorption peak of Hu at the 1 630 cm^-1 band increased by 0.33% in the corn growing season. The peanut shell derived biochar used in this experiment was relatively higher in content of aromatic groups than the wood derived biochar. Hu increased in condensation degree, while decreased in oxidation degree in Treatment T2, however, it acted reversely in Treatment T3.[Conclusion] Biochar was conducive to increasing of the content of HS, especially Hu, and it increased Hu in condensation degree and decreased Hu in oxidation degree with the experiment going on. Application of biochar could also increase relative contents of methylene and methyl in HS. The biochars derived from different materials are different in chemical structure, which in turn affects chemical composition of the HS in the soil.