As the relationship between the spatial distribution characteristics and physicochemical properties of nitrogen in lagoon soil and sediment is still unclear, we obtained and systematically analyzed soil and sediment samples from the surroundings of a lagoon in the Shamei Inland Sea, Qionghai City, Hainan Province. The spatial distribution of nitrogen forms was investigated, and the soil physicochemical properties that predominantly influenced the nitrogen distribution were characterized. The results were as follows: (1) There are differences in nitrogen content in the soil and sediment around the Shamei Inland Sea. The total nitrogen levels in the Shamei Inland Sea were low, and organic nitrogen was dominant. The soil samples showed higher organic nitrogen concentrations than the sediment samples. Thus, the characteristics of dispersion in offshore waters differ from those in lagoons. The average contents of nitrate, ammonium and nitrite in the soil around the lagoon were higher than those in the sediment, and they were especially high in the lagoon mouth area. The nitrate content was greater in the estuary area in the northwest of the lagoon, and the contents of ammonium and nitrite in the estuary area in the south of the lagoon were the highest. (2) The changes in the basic physical and chemical properties of the soils and sediments in the Shamei Inland Sea area have an important coupled effect on the enrichment, preservation and mineralization of nitrogen. Through redundancy analysis (RDA), the explanatory degree of the organic matter content in relation to the nitrogen content was determined to be approximately 17.2%, contributing 80.9% of all nitrogen. The organic matter content, cation exchange capacity and clay content were positively correlated with the nitrogen content, indicating that changes in the basic physical and chemical properties of the soil and sediment had an important impact on nitrogen enrichment, preservation and mineralization processes. The total nitrogen and the organic matter content, cation exchange capacity and clay content of the soil and sediment were positively correlated. The high proportion of nitrate in the soil, the high proportion of ammonium in the sediment, the heavy texture of the sediment, poor soil ventilation and weak nitrification were related.