Abstract: With the extensive application of sand-silt mixtures in engineering construction, studying its mechanical properties has significant meanings to construction of sea dykes and breakwater. Although there is a substantial researches on the mechanical properties of sand-silt mixtures, the effects of relative density (D_r) and fines content (C_F) on the phase transition characteristics of sand-silt mixtures still require further investigation. Using the GDS static triaxial instrument, we carried out a series of consolidation undrained shear tests on the mixtures with different D_r and C_F. It is found that D_r has a notable effect on the pore water pressure (u_e), phase transition strength (S_PT) and effective internal friction angle (φ′_PT) in the phase transition state of the sand-silt mixtures; C_F has a similarly significant effect on u_e and S_PT in the phase transition state of the sand-silt mixtures, but has almost no effect on the phase transition angle of the sand-silt mixtures. In addition, a composite compaction state parameter e×D_r was introduced to establish a method to evaluate the S_PT of sand-silt mixtures. This method demonstrates a good predictive capability and has practical engineering application values.