The typical Suaeda salsa marsh in the newly created marshes of the Yellow River estuary was selected as study object. The variations of phosphorus (P) fractions in soils during the growing season were investigated by in situ nitrogen (N) import experiment which included four treatments (NN, no N enrichment; LN, low N enrichment; MN, middle N enrichment; and HN, high N enrichment). Results showed that the contents of non-available P (HCl-P_i and Residual-P) were the main part of total phosphorus (TP) in soils of different treatments accounting for 87.75%-90.04%, while both the proportions of available P and moderately available P in TP were very low with 4.81%-5.58% and 5.14%-6.57%, respectively. Compared to NN, the contents of available P in MN and HN increased by 9.16% and 12.44%, respectively, whereas the values of moderately available P in LN, MN, and HN increased by 2.25%, 6.92%, and 24.24%, respectively, indicating that N enrichment generally increased the contents of available P in soils. By comparison, the contents of non-available P in LN, MN, and HN decreased by 3.08%, 3.08%, and 5.22% compared to those in NN, respectively. Although soil nutrients, moisture, salinity and acid-base status in different treatments were the key factors affecting the amounts of P factions, with increasing of N input, the types of nutrients affecting P fractions changed significantly from NN and LN which were mainly affected by N and sulfur (S) to MN and HN which were mainly affected by P. This study found that exogenous N import not only might affect the transformations among P fractions, but also might influence the variations of P fractions in soil by altering the growth rhythm of plants and the nutrients and acid-base status of soils.