<p>In this study, an innovative experimental methodology for determining the plastic strain increment direction of sand is proposed. A high-precision fitting function is constructed to reconstruct the discrete plastic strain path data points into a continuously differentiable smooth curve, whose gradient direction precisely characterizes the plastic strain increment direction. Complementing this approach, a matching procedure of continuously calculating elastic strain is developed to weed out the elastic constituents from the collected total strain. Drained conventional triaxial compression tests are performed on Fujian standard sand specimens, and the void ratio and the stress state-dependence of the macroscopic mechanical behavior is systematically revealed. The performance of the proposed method is evaluated through the comparison with conventional finite difference method in characterizing plastic strain increment direction. The experimental data are input into the proposed method to construct the continuous evolutionary trajectories of first counterclockwise followed by clockwise reorientation for the plastic strain increment direction throughout loading. Further analysis elucidates the regulatory mechanisms of the void ratio and the stress state on the plastic strain increment direction, where reduced void ratio amplifies leftward deviation of the direction vector, whereas elevated confining pressure induces more rightward deflection. This research resolves the mismatch issue between the discrete test data and the theoretical differential nature of the plastic strain increment direction, providing a robust theoretical foundation and experimental validation for constitutive modeling of sand.</p>