Mechanical behavior of sulphoaluminate cement (SAC) mortar mixed slag (SL), fly ash (FA) and combined admixture were studied in the study, at the same time, hydration products and micro-structure were analyzed with XRD and SEM for making clear hydration mechanism. Test results show that early period strength of SAC mortar decreased, and strength contribution rate of admixture is combined admixture (slag + fly ash) ＞ slag ＞ fly ash on the same content due to the combined admixture is more beneficial to accelerate hydration. Yet strength increase of SAC mortar mixed admixture after cured 60d is not obvious, which was not large different with pure SAC mortar. A large amount of ettringite and gel produced during 1d period of SAC paste, but with the hydration carrying on, diffraction peak of hydration products (ettringite and gel) and unhydrated calcium sulphoaluminate change little, which means hydration degree at 1d is higher. Diffraction peak of ettringite and gel at different curing period is lower when mixed admixture in matrix, the amount of ettringite and gel increased and unhydrated calcium sulphoaluminate decreased with hydration carrying on, which means early hydration speed slow down when introduction of admixture into SAC matrix.