Abstract: The red mud (RM) and phosphogypsum (PG) were used to stabilize mud with traditional cement in this study, aiming to utilize potential chemical activity of industrial by-products in cement-clay cementitious system. The unconfined compressive strength (UCS) tests, consolidated undrained triaxial compression tests, hydraulic conductivity tests and microscopic tests were conducted to examine the evolution of mechanical behavior of stabilized clay with RM/PG proportions. The microscopic coupled effects in stabilized clay were clarified. The results show that addition of RM and PG improves the mechanical behavior of stabilized clay significantly, and the synergistic effects between RM and PG are remarkable. An optimal proportion of R7.5P2.5 can increase the UCS of stabilized clay by 0.8~1 times. The alkali-activation effects of RM contributes to the dissolution of clay mineral and the promotes pozzolanic reactions, resulting in stronger cementation effects of soil particles and larger cohesion. The PG contributes to the generation of ettringite and fills pores, resulting in a larger frictional angle. In addition, the failure strain of stabilized clay increases significantly with addition of PG, indicating a stronger capacity in plastic deformation. The coupled alkali-activation effects, pore-filling effects and cementation damage effects exist in RM-PG-cement stabilized clay. The evolution of mechanical behavior is consistent with microscopic coupled effects.