The　autogenous　shrinkage　of　alkali-activated　slag　(AAS)　is　significantly　higher　than　that　of　ordinary　Portland　cement　(OPC).　The　higher　risk　of　concrete　cracking　due　to　autogenous　shrinkage　is　a　critical　drawback　to　wider　use　of　this　promising　alternative　binder.　The　effect　of　CaO　content　on　the　autogenous　shrinkage　of　AAS　mortar　was　investigated.　The　autogenous　shrinkage　of　AAS　mortars　was　determined　by　comparator.　The　pore　structure　of　the　pastes　was　determined　by　mercury　intrusion　porosimetry.　The　hydration　products　of　the　pastes　were　determined　by　Fourier　transform-infrared,　thermogravimetric　analysis,　X-ray　diffraction,　and　Si-29　solid-state　magic-angle　spinning　nuclear　magnetic　resonance.　The　results　show　that　the　amount　of　portlandite　increases　as　CaO　content　increases.　CaO　in　the　paste　causes　the　partial　replacement　of　C-S-H(I)　(low　stiffness)　by　C-S-H(II)　(high　stiffness).　The　hydration　reaction　of　AAS　is　inhibited　by　the　addition　of　CaO.　The　increase　of　polymerization　degree　of　C-(A-)S-H　and　rearrangement　of　C-S-H(I)　during　hydration　are　inhibited　by　the　addition　of　CaO,　and　micropores　closure　is　also　inhibited.　Therefore,　the　autogenous　shrinkage　of　AAS　mortar　decreases　with　the　increase　of　CaO　content.