To investigate the mechanism of the effect of calcium oxide and sodium carbonate activated slag on the autogenous shrinkage of alkali-activated cement, an alkali-activated slag martar (AM) was prepared using calcium oxide and sodium carbonate (molar ratio of 1∶1) as the combined activator. The effect of combined activator Na₂O equivalent (i.e. the by weight ratio of Na₂O produced by the reaction of the combined activators to slag, i.e. 2.5%, 4.5%, 6.5% and 8.5%) on the autogenous shrinkage of AM was investigated. The hydration products and microstructure were characterized by X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, and nuclear magnetic resonance. The results indicate that with the increase of Na₂O equivalent, the increase of water consumption due to the reaction of activators and pore structure refinement leads to the increase of pore pressure, and the amount of absorbed Na+ increases due to the increase of amount of Si⁴⁺ in C-(A)-S-H replaced by Al³⁺, resulting in the increase of C-(A)-S-H slippage. The increase of hydration degree induces the increase of the amount of hydration products, resulting in the increase of the autogenous shrinkage. AM with Na₂O equivalent of 6.5% is the best group with its mechanical properties higher than those of ordinary Portland cement mortar (OM), but its autogenous shrinkage greater than that of OM due to its lower crystal content and denser pore structure.