As one of the most important hydration products of Portland cement,C-S-H gel has a great effect on macro-performance,physicochemical properties of cement stone and concrete.However,because the chemical compositions and structure of C-S-H gel are not well established,it is very difficult to clarify it in the field of cement chemistry till now,especially by the quantitative method.In this paper,a modified method based on Olson's water absorption method has been proposed and a semi-quantitative study on C-S-H gel in cement by this method has been done.