The high water material based on sulphoaluminate cement was modified by the gel of hydrolyzed aluminum sulfate. The relationship between strength and microstructure of modified high water material was studied by such modern way as X - ray diffraction ( XRD) ,scanning electron microscope ( SEM) ,and mercury porosimeter,etc. The results showed that the main composition of hydrate was ettringite after gel - modification of high water material. The compressive strength of structure was 3. 5MPa at 1 day,and reached 7. 8MPa at 28 days. The gel - modification with aluminum sulfate could promote the formation of ettringite,improve the micro pore structure of hydrate,and increase the compactness and strength of the high water material. The high water material after gel - modification could be used as a kind of filling material with good performance.