Abstract: In order to scientifically predict the strength and deformation characteristics of natural foundations and large-area fill foundations, a comprehensive and systematic study was conducted on the mechanical properties of transversely isotropic unsaturated remolded loess through theoretical analysis and systematic indoor experiments. Firstly, based on the strength theory of spatial sliding surfaces and considering the sedimentation angle, transverse isotropy, and unsaturated characteristics of layered foundations, the matrix suction and transverse isotropy parameters are introduced into the Lade Duncan strength criterion, and the strength criterion for transverse isotropic unsaturated loess is established; Secondly, various unsaturated triaxial and true triaxial tests with different stress paths were designed to determine the relevant parameters in the three-dimensional strength criteria for transversely isotropic unsaturated loess; Thirdly, the strength, deformation, and water content variation characteristics of transversely isotropic unsaturated remolded loess were revealed; Finally, the strength criterion proposed in this article was compared and analyzed with the verification test results, as well as the Lade Duncan strength criterion and Mohr Coulomb criterion, to preliminarily verify the rationality and applicability of the transverse isotropic unsaturated loess strength criterion based on spatial sliding surface variation. The research results can provide theoretical support and scientific basis for the design of natural layered foundations and large-scale fill engineering, and are of great significance for promoting the strength theory research of unsaturated soil mechanics.