Analyzed the change of hydrate in autoclaved fly ash lime system with acid dissolve,X-ray Diffraction(XRD),Infrared Spectroscopy(IR) and Scanning Electron Microscope(SEM).The experimental result showed,hydrogarnet change endlessly in autoclaved condition.Hydrogarnet translate into calcium silicate hydrate,and the hydrate has two types: fine fibrous hydrate and plate high alkali hydrate-bearing Al.The hydrate is fine fibrous hydrate when [Ca~(2+)] is lacking in the system.The hydrate is plate high alkali hydrate-bearing Al and could form well crystallized tobermorite when hydrogarnet break down largely.