The influences of the monocyclic aromatic high range water reducers on cement hydration were evaluated from four factors, such as hydration heat, hydration products, pore structure, microstructures of cement pastes. With measurement of hydration heat by the TAM Ai, it was found that monocyclic aromatic high range water reducer may delay early hydration and significantly decreased early hydration heat. And the monocyclic aromatic high range water reducers tended to increase porosity and pore volume of cement pastes with MRI analysis, while pore size distribution did not change remarkably. With analysis of XRD、TGDTA and SEM, it was shown that the monocyclic aromatic high range water reducers could restrain the production of Ca(OH)2 and Hydrated Calcium Silicates in the hydration process without affecting the end products, and refine the hydration products, such as Ca(OH)2, CSH gel and ettringite crystals.