To investigate the cementation mechanism of mixed red mud, the variations of the chemical composition, mineral composition and microstructure of mixed red mud are analyzed by means of XRD, XRF and SEM tests during the dehydrating process; and the variation of cementation strength during dehydrating is also discussed. Based on this, the triaxial shear tests under consolidation drained condition are carried out to determine the mechanical properties of five mixed red mud samples of different dehydration ages, and to reveal the influence of formation of cemented strength. The results show that the cementation strength mainly derives from the hydrating reactions of hydraulic materials like cement and carburizing reagents of activating oxide in mix red mud, and the significant growth of the strength appear within dehydration age of 70 days. It is also shown that the shear strength of mixed red mud have direct correlation with cementation strength, especially the cohesive strength. To investigate the impact of the structure on the deformation and strength of the mixed red mud, we introduce a structural parameter, which is defined as the ratio of principal stress difference of dry sample with different dehydration ages under triaxial compression to that of original sample. A good correlation between the structure and deformation and strength is found based on the structural parameter.