In order to improve dimensional stability of cementitious materials, synergistic effect of shrinkage reducing admixtures (mixture of glycol ethers and siloxane, SRA) and MgO-based expansive agent (MEA) burned at 850 °C, 950 °C, 1050 °C and 1200 °C for 1 h on the deformation of cement paste was investigated while being cured in water of 20 °C, 40 °C and 60 °C, sealedly by polyethylene sheet at 20 °C, 40 °C and 60 °C as well as in simulated using environment. The results illustrated that combined use of SRA and MEA (burned at 850 °C and 950 °C) could compensate the shrinkage of cement paste effectively, and MEA also could make up for the shortage of SRA whose shrinkage-reducing ratio decreased at later age. In the simulated using environment, the paste both with SRA and MEA (burned at 850 °C and 950 °C) was in micro-expansion condition, and the average deformation increased slowly till 240d, and the deformation amplitude reduced to the half of the control sample. Then hydration of MEA in the present of SRA was examined by gravimetry. The results indicated that the presence of SRA retarded the hydration of MEA, but with prolonged curing, the hydration degree of MEA with or without SRA tended to be the same.