The total alkali content and soluble alkali content in cement paste were determined.The change of fluidity and its time-depended lose of cement paste were investigated with the modi-(fications of the) content of soluble alkali in cement paste and the dosage of aminosulphonate based superplasticizer.The influence of the content of soluble alkali in cement paste on the compatibility of cement and aminosulphonate based superplasticizer was discussed.The researching results show that the fluidity of cement paste is obviously affected by the content of soluble alkali in cement paste and the dosage of aminosulphonate based superplasticizer together.The content of soluble alkali in cement paste is lower than the optimal one.Addition of adequate quantity of soluble alkali benefits increase of the fluidity and decrease of its time-depended lose of cement paste when the dosage of aminosulphonate based superplasticizer is lower than its saturated amount.There is little effect for the soluble alkali content in cement paste when the dosage of aminosulphonate based superplasticizer is greater than its saturated amount.