To explore the effects of different planting densities of oat on the physical and chemical properties of secondary salinized farmland soil, three oat return treatment groups with low density (YMD), medium density (YMZ), and high density (YMG) were set up in a typical secondary salinized farmland in Beiwugegutu Village, Huatugula Town, Zuozhong Banner, Tongliao City, Inner Mongolia Autonomous Region. The bare land without oat planting was used as the control (CK). The oat was returned to the field when it grew to the beginning of July. Soil samples from the 0-20 cm layer were collected three days before and three months after the return of the oat, and the pH value, exchangeable sodium percentage (ESP), anion and cation composition, organic matter content, and available nitrogen, available phosphorus and available potassium were determined. The results showed that the return of oat significantly improved the physical and chemical properties of salinized soil. Among them, the high-density treatment (YMG) showed a more significant improvement effect compared with the medium-density (YMZ), low-density (YMD) and control (CK) groups. Compared with CK, the return of oat at different densities could effectively increase the soil organic matter content by nearly 5.00 g/kg. Compared with CK, YMG simultaneously reduced the soil pH value and ESP by 0.77 and 8.18%, respectively, and the effect of high-density treatment was the most obvious. The analysis of eight major ions showed that the concentrations of K?, Na?, Cl?, HCO?? and CO?2? decreased, and YMG decreased by 0.44, 0.45, 0.19, 0.19 and 0.13 g/kg respectively compared with CK; while the contents of Ca2?, Mg2? and SO?2? increased by 0.19, 0.12 and 0.18 g/kg respectively. In terms of nutrient indicators, the available nitrogen content increased by 21, 28 and 44.33 mg/kg respectively under YMD, YMZ and YMG treatments compared with CK; the available phosphorus content increased by 5.57, 7.09 and 7.45 mg/kg respectively; and the available potassium content increased by 22.61, 26.59 and 34.56 mg/kg respectively. In conclusion, planting and returning different densities of oat in secondary salinized farmland can effectively alleviate the degree of soil salinization and significantly improve the soil fertility level.