【Objective】 This paper was oriented to study changes in content and composition of soil soluble organic nitrogen (SON) and their influencing factors in paddy field under long-term fertilization varying in scheme, in an attempt to provide certain scientific bases for revealing chemical essence and ecological functions of SON in paddy soil and rationalizing fertilization schemes. 【Method】A 33-year long-term fertilization experiment, designed to have four treatments: CK (no fertilizer), NPK (chemical fertilizer), NPKM (chemical fertilizer + cattle manure) and NPKS (chemical fertilizer + straw) was selected as object of the study. With the aid of automatic amino acid analysis and Fourier infrared spectrum analyzer coupled with redundancy analysis (RDA) method, effects of the long-term fertilization treatments on content and composition of SON and main influencing factors in the paddy soil were explored. Soil SON was extracted with hot water. Portions of soil samples from the treatments, 5.0 g each, were put into flasks with stopper, separately, with 25 ml of deionized water each, incubated at 70 ℃ for 18 h, shaken for 5 min on a shaker, and filtered through a 0.45 um filter membrane. The extracts were analyzed for total soluble N (TSN) with the high-temperature catalytic oxidation method and a TOC analyzer (Shimadzu, Japan) and for concentrations of soil inorganic nitrogen (SIN) (NH₄⁺–N, NO₃^-–N and NO₂^-–N) with a continuous-flow analyzer (Systea, Italy). The concentration of SON was calculated as the difference between TSN and the sum of SIN in the extracts. 【Result】Long-term fertilization significantly affected content and composition of the SON in the paddy soil relative to treatment. SON in the treatments accounted for 46.16%～62.45% of the TSN in the paddy soil under the long-term fertilization , making up an important component of the soil soluble nitrogen storage in the paddy field. The content of soil SON in Treatment NPK, NPKM and NPKS was 23.49%, 58.70% and 106.30% respectively higher than that in CK, which indicates that Treatment NPKS, long-term application of chemical fertilizer plus straw is more conducive to accumulation of SON in the paddy soil under the premise that the same amount of NPK is applied. The content of free amino acid nitrogen in Treatment NPK, NPKM and NPKS was 32.27%, 84.42% and 95.21% respectively higher than that in CK, accounting for 31.94%～39.23% of that of the total SON, separately. The free amino acids in the treatment exhibited a decreasing order of neutral amino acids > acidic amino acids > basic amino acids in content, and were dominantly composed of threonine, serine, sarcosine, glycine, alanine and isoleucine. The contents of acidic and basic free amino acid nitrogen were significantly increased in Treatments NPKM and NPKS. The content of soluble protein nitrogen in Treatment NPK, NPKM and NPKS was 20.83%, 70.00% and 95.83% respectively higher than that in CK, accounting for 4.72%～5.32% of that of the total SON, separately, which indicates that application of chemical fertilizer in combination with organic manure or straw, especially Treatment NPKS, can significantly increase the content of soil soluble protein nitrogen. Functional groups of the soil soluble nitrogen in Treatments NPKM and NPKS, especially in the latter, are reflected as amide compound featured infrared absorption peak at 3 400, 1 630～1 650, 1 400～1 460 and 1 310 cm^-1 along the spectrum. RDA results show that SON varied significantly with treatment in content and composition. SON in Treatments CK and NPK was mainly affected by soil bulk density, while that in Treatments NPKM and NPKS was by organic matter, total nitrogen, microbial biomass nitrogen, protease and glutaminase.【Conclusion】 SON is closely related to fertilization treatment in content and composition, and addition of organic manure can significantly increase the contents of soil SON, acid and alkaline free amino acids and soluble protein nitrogen. The increased SON in Treatments NPKM and NPKS is mainly small molecule easily mineralized nitrogen and large molecule nitrogen of unknown forms. Free amino acids, amides and soluble proteins are the main components of SON in paddy soil. Bulk density, organic matter, total nitrogen, microbial biomass nitrogen, protease and glutaminase are the main factors affecting content and composition of SON in paddy field, but vary in degree with treatment.