【Objective】Returning straw into soil and applying composted straw and biochar derived from straw are effective ways of increasing soil organic carbon (SOC). 【Method】Samples of saline soil were collected from the Yellow River Delta and incubated for 30 and 180 days in lab in order to study effects of application of straw, compost and biochar on composition and structural characteristics of humic substances in the soil. 【Results】Results show that the practices increased humin (Hu) more than humic acid (HA) and fulvic acid (FA). The content of Hu increased from 5.48 g·kg^-1 increased to 11.20 g·kg^-1、16.66 g·kg^-1 and 20.60 g·kg^-1, after 180 days of incubation. In addition, the content of HA increased to 1.36 g·kg^-1 in the plot applied with compost plus biochar, while in the plot of straw returning, the content of FA decreased from 3.77 g·kg^-1 to 3.32 g·kg^-1, after 30 days of incubation. Positive correlations were observed between the contents of Hu and SOC in the plots incubated for either 30 (R² = 0.84, P< 0.001, n = 10) or 180 (R² = 0.98, P< 0.001, n= 10) days. Organic carbon (OC) increased mainly in Hu, especially in the plot applied with biochar. Both application of compost or returning of straw were conducive to increasing HA and FA, and application of compost plus biochar was more significant in the effect of increasing HA. The content of Hu was mainly affected by the ratio of biochar in the plots applied with biochar plus straw. Application of straw, compost or biochar was in favour of accumulation of aliphaic groups of FA, and good to accumulation of aliphaic groups and carbohydrate in HA at the initial stage in the plots applied with straw or compost, especially with compost. Application of biochar was good to increasing aromatic groups in HA, while application of straw or compost, especially the latter, was to increasing aliphatic groups and polysaccharide at the initial stage of incubation. But afterwards, aliphatic groups deceased, while polysacharide went on increasing with the incubation lasting, regardless of treatment. 【Conclusion】In summary, biochar mainly increases the content of Hu, while compost plus biochar is in favor of increasing HA. However, straw increases the content of FA at the initial incubation stage. The ratio of biochar in the plots of applied with biochar plus straw or compost affects the content of Hu, aromatic functional groups of HA and aliphatic groups of FA.