Abstract: The soil volume in triaxial tests is an essential parameter to characterize stress and strain behaviors of soils. In recent years, more and more image-based methods have been developed for the triaxial tests on deformation measurement of soils. However, the measured results are only the relative volume change of specimens, not their absolute volume. In order to measure the absolute soil volume in triaxial tests, the photogrammetry principle, optical ray tracing and the least square optimization are used to measure the three-dimensional position of point clouds on the soil surface. The absolute volume of the soil specimens during triaxial tests is then obtained through triangulation, end cut and interpolation based on the obtained point clouds. Meantime, a software package, GeoTri3D, is developed to implement the above-mentioned steps and post processing of the measured results. In order to validate the proposed method, triaxial tests are conducted on a standard steel cylinder, a saturated sand specimen, and three soil specimens. The results obtained from the validation tests indicate that the proposed method can successfully and accurately capture the absolute volume of the soil specimens during triaxial tests at different time of interests. In addition, based on the analysis on the triangular meshes, the strain contour plots can be generated, which facilitates the visualization of the deforming process of soils during triaxial tests.