In order to study the variation of shear strength of unsaturated sandy soils and establish an effective prediction model, taking sandy soils in Nanchang area as the representative, the mechanism of the influence of water content and dry density on the shear strength of unsaturated sandy soils was investigated. The results show that under different water content and dry density conditions, the shear stress-shear displacement curves of unsaturated sandy soils are all of softening type, showing a four-stage nonlinear law of change, which presented a decreasing variability with the change of water content and dry density. The shear strength of unsaturated sandy soils decreases with increasing water content and saturation, which increases with increasing dry density. The cohesion of unsaturated sandy soils decreases and then increases with increasing water content, increasing approximately linearly with increasing dry density. The angle of internal friction increases and then decreases with increasing moisture content, and the change in the coefficient of internal friction resulting from the change in dry density is small, within about 25%. Based on the Moir-Coulomb strength criterion, regarding water content and dry density, a nonlinear regression model of shear strength of unsaturated sandy soils was constructed, which could provide a reference for the selection and prediction of engineering geological parameters.