<p>In comparison with normally consolidated soft ground, consolidating soft ground often displays inferior engineering properties, which have not been thoroughly investigated yet. This study aims to investigate the undrained shear characteristics of consolidating soft soil under both compression and extension test conditions. A series of undrained triaxial compression and extension tests are conducted on reconstituted kaolin clay under different degrees of consolidation. The results indicate that the undrained stress–strain curves, the evolution of excess pore water pressure, and the undrained stress paths for both normally consolidated and consolidating soft soils exhibit a similar pattern. It is also found that consolidating soft soil also follows the Mohr–Coulomb failure criterion, which can be expressed in terms of effective consolidation pressure and is considered to be independent of degree of consolidation. The primary difference between normally consolidated and consolidating soft soils lies in the initial tangent modulus, which decreases as the degree of consolidation decreases. Subsequently, a modified Duncan–Chang constitutive model is developed to accurately approximate the measured stress–strain curves of consolidating soft soil. Finally, the proposed model is validated by the experimental data, demonstrating its capability to effectively capture the influence of the consolidating state on shear characteristics.</p>