Geopolymer, a novel cementitious material of superb corrosion resistance and quick setting and bigstrength in early stage, is applied to offshore and coastal projects. It is also, however, subject to cracking and large volume shrinkage which limit its engineering application. In this paper, the effects of MgO of high activity (60 s), medium activity (150 s) and low activity (220 s) on the volumetric shrinkage and microstructure of hardened geopolymer pastes were examined respectively. Results revealed remarkable influence of MgO activity on geopolymer performance: MgO of high activity could improve the early-stage strength of hardened geopolymer pastes by enhancing hydration heat release, and the early shrinkage value of the hardened pastes were the lowest; on the contrary, MgO of medium activity and low activity postponed the hydration heat release, and alleviated the volumetric shrinkage by acting with water in the geopolymer.