There covers very thick soft clay in Zhujiang area. For a project of this area, the cement deep mixing method is used to improve the clayey ground. However, the soft clay is very thick and it is lack of experience, so we need to study the strength of cement mixing soil: after taking soil sample in situ, we start our test on the strength of cement mixing soil. Taking the cement amounts as 8%, 10%, 12%,14%,16% and 18% respectively, and water cement ratios as 0.9, 1.1 and 1.3, we analyze the strength of cement mixing soil at different ages. The results show that even under the condition of 18% cement amount and 0.9 water cement ratio, the strength of cement mixing soil at 90 d age is less than 1.0 MPa, which is lower than normal. After mixing fly ash and admixture of UNF-5, the strength of cement mixing soil is increased to 1.4 MPa.Therefore, admixtures must be mixed into the cement mixing soil to increase the active strength.