Abstract: In order to scientifically predict the strength and deformation characteristics of large-area fill foundations, a comprehensive and systematic study is conducted on 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 transverse isotropy, and unsaturated characteristics of layered foundations, 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 are designed to determine the relevant parameters in the three-dimensional strength criteria for transversely isotropic unsaturated loess; Thirdly, strength, deformation, and water content variation characteristics of transversely isotropic unsaturated remolded loess are revealed; Finally, the strength criterion proposed in this article is 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 large-scale fill engineering, and are of great significance for promoting the strength theory research of unsaturated soil mechanics.