【Objective】Soluble organic nitrogen (SON) is one of the most active components of the soil nitrogen pool in farmland and plays an important role in nitrogen transformation and ecological environment security. As SON is highly mobile, it is prone to get lost with runoff or leaching, and thus, and hence affect water quality. 【Method】To provide a theoretical basis for rational fertilization in paddy fields, prevention and control of non-point source nitrogen pollution from paddy fields and improvement of the theory of nitrogen cycling, a field experiment, designed to have the following treatments, all equivalent in nutrient content, i.e. CK (application of chemical fertilizer only), CMV1 (application of chemical fertilizer plus incorporation of milk vetch at 15 000 kg·hm^–2); CMV2 (application of chemical fertilizer plus incorporation of milk vetch at 30 000 kg·hm^–2); and CMV3 (application of chemical fertilizer plus incorporation of milk vetch at 45 000 kg·hm^–2), was carried out to explore dynamics, migration and loss of SON and dissolved organic nitrogen (DON) in grey-mud field soils. Soil samples from the treatments were analyzed for SON, respectively, using the hot water extraction method. SON concentrations were calculated by deducting TSN from the sum of SIN in the extracts. 【Result】During the growing period of rice, SON content in the 0-60 cm soil layer varied in the range from 2.09 to 22.32 mg·kg^–1, showing a trend of “increasing - decreasing - increasing - decreasing to stable”. In the treatments the SON concentration was 140.95% and 364.44%, higher in the 0-20 cm soil layer, respectively, than in the 20-40 cm soil layer and the 40-60 cm soil layer, and 92.75% higher in the 20-40 cm soil layer than in the 40-60 cm soil layer. It is quite obvious that incorporation of Chinese milk vetch during the growth period of rice is beneficial to accumulation of SON in the soil (0-60 cm). In SON concentration in the 0-60 cm soil layer during the rice growth period, Treatment CMV1, CMV2 and CMV3 was 5.57%, 10.11% and 21.39%, respectively, higher than CK. Total loss of DON from the grey-mud fields under different fertilization treatments ranged from 18.33-58.55 kg·hm^–2, accounting for 46.52%-50.16% of the TSN. Of the total loss, 3.77-37.85 kg·hm^–2 was attributed to runoff, while 14.5-18.02 kg·hm^–2 to leaching. Moreover, leaching of DON between soil layers was delayed to some extent. Compared with CK, Treatment CMV1, CMV2 and CMV3 was 16.90, 31.09 and 37.52 kg·hm^–2, respectively, lower in DON loss.【Conclusion】Incorporation of milk vetch can increase the accumulation of SON in the 0-60 cm soil layer of the grey-mud field and promote the migration of SON. However, the effects decline with soil depth. DON is one of the important forms of nitrogen loss in paddy fields. Incorporation of milk vetch can reduce DON loss and then mitigate non-point source N pollutant from the paddy fields. The environmental impact of the loss of DON from paddy fields is worthy of attention. If only the loss of inorganic nitrogen with leaching of paddy soil solution is considered, the total nitrogen loss from paddy fields will be underestimated.