Cement and all kinds of admixture have been used in the treatment engineering of soft soil foundation in coastal cities to improve soil strength and sustain the demands of engineering construction. In engineering practice, soil composition varies because of different origins and formation environments. Thus, cement consolidated soil likely exhibits different strength characteristics when different admixtures are added. In this study, soft soil obtained from Fuzhou, Zhuhai, and Yingkou was used as test samples. Physical properties, soluble salt content, organic matter content, and other basic parameters of the soil were analyzed. Indoor tests were also performed to simulate the consolidating process of soft soil with different admixture. Uniaxial compressive test was also performed to measure mechanical index. The combined effects of cement hydration reaction with different organic matter composition, granularmetric and water characteristics were considered to determine the effect mechanism of cement consolidated soil with different admixtures. Results showed that the soft soil of the three areas contained high amounts of organic matter and soluble salt. The uniaxial compressive strength increased when early-strength agent and water-reducing agent were used. However, the increased amplitude varied with different early-strength agents. The optimum effect was obtained when CaSO4 was added, and the sample was inseparable with high amounts of fulvic acid and humic acid. The same additive agent resulted in uniaxial compressive strength with low moisture content and high clay content. Thus, the basic properties of soft soil and its organic matter content should be considered in selecting the optimum admixture during cement consolidation in engineering.