Relationship between the autoclave-treated expansibility of cement mortar, one-graded concrete and the autogenous volume deformation of three-graded MgO-mixed concrete at six-year age for the three-graded fly ash concrete commonly used in water retaining dams of water conservancy and hydropower engineering were studied based on the methods such as autoclave-treated test, autogenous volume deformation observation, mercury intrusion porosimetry, scanning electron microscopy, energy-dispersive X-ray spectroscopy and so on. The results indicate that there is no relevance between the autoclave-treated expansibility and the autogenous volume deformation. The current standard to determine the limit MgO content in dam concrete is based on the inflection point of the autoclave-treated expansibility curve or the autoclave-treated expansion ratio at 0.5% of cement mortar and one-grade concrete samples. This standard can ensure the safety operation of a concrete dam but the actual expansive deformation of MgO-mixed concrete cannot be reflected.