In order to investigate the seasonal dynamics of soil soluble nitrogen after returning farmland to forest in loess hilly region, the Hippophae rhamnoides forestland and Armeniaca vulgaris Lam. forestland in Yongning Town of Zhidan County were taken as the research object. Soil samples were collected in 0-10 cm, 10-20 cm and 20-30 cm soil layers, and seasonal dynamic variation of the density and proportion of soil soluble nitrogen component were analyzed. The results showed that the measures of returning farmland to forest significantly increased the density of soil nitrate nitrogen, ammonium nitrogen and soluble organic nitrogen, especially in the 0-10 cm soil layer. There were significant differences in the above indexes among different soil layers and between H. rhamnoides forestland and A. vulgaris Lam. forestland. On the whole, nitrate nitrogen density varied significantly with seasons, the value was the highest in March, declined from March to June, increased from June to September, declined from September to November, and was the lowest in November. The ammonium nitrogen content in 0-30 cm soil layer varied little. The soluble organic nitrogen content varied significantly with seasons, and the highest and lowest value was found in June and November, respectively, and the content increased from March to June, declined from June to September and rised from September to November. The seasonal variation of soluble organic nitrogen density was opposite to that of nitrate nitrogen. The content of soluble nitrogen in H. rhamnoides forestland was higher than that in A. vulgaris Lam. forestland. The soluble nitrogen content of sunny slope was higher than that of shady slope. In 0-30 cm soil layer, the soluble nitrogen was mainly soluble organic nitrogen, accounting for 73.3%~99.0%, followed by nitrate nitrogen accounted for 0.9%~24.3%, and the proportion of ammonium nitrogen was the least, which was below 2%. The conversion of farmland to forest significantly increased the density of soluble nitrogen in 0-10 cm soil layer. The soluble nitrogen density increased differently in different forestlands, the value in H. rhamnoides forestland was higher than that in A. vulgaris Lam. forestland.