In this paper, a triple large-scale biaxial tensile test system for geosynthetics developed by the authors was used to study the tensile mechanical properties of warp-knitted polyester (PET) geogrids. In-isolation tensile (in air) tests with various strain rates were conducted to investigate the effects of tensile modes (uniaxial and biaxial tension) on the tensile mechanical properties of warp-knitted PET geogrids. To evaluate the influences of normal stress and confined soil types (sand and gravel) on the tensile load-strain characteristics of warp-knitted PET geogrids under uniaxial and biaxial tensile loading, strain rate-controlled tensile tests in soil were also carried out. The results demonstrated that the low strain rate leads to low tensile load and secant tensile stiffness of geogrids in-isolation tensile tests. The biaxial in-isolation tensile tests mobilized a lower tensile load throughout the tensile process. The constraint of soil types and the application of normal stress increased the tensile load and secant tensile stiffness of geogrids. In general, the confined soil reduces the impact of uniaxial and biaxial tensile loading on the tensile tests. Geogrids embedded in sandy soils showed improved mechanical properties.