The effect of different activated MgO on the drying shrinkage performance of alkali-slag-metakaolin based geopolymer（ASM geopolymer） was investigated. The internal mechanism of influence was revealed by X-ray diffractometer（XRD）， scanning electron microscope（SEM）， mercury porosimetry（MIP） and other microscopic test methods. The results show that with the increase of MgO activity， the drying shrinkage of ASM geopolymer decreases at first and then increases， and MgO with moderate activity can reduce the drying shrinkage of ASM geopolymer by about 15.0%. The generated Mg-Al hydrotalcite and magnesium silicate hydrate（M-S-H） gel effectively fill the pores， result in reducing drying shrinkage and increasing structural compactness.