The marine soft soil in Tianjin coastal region has high water content, low strength, high compressibility and low pH. It can't meet the needs of engineering construction and has to be improved. One of the soil improvement methods is to mix feasible alkalic additive sodium hydroxide or sodium carbonate into the whisking cement soil. This method can improve the strength of pile cement soil and the bearing capacity of composite foundation, and save a great deal cement. In this paper, the marine soft soil is mixed with 10% cement added with 0.5% NaOH and 0.5% Na2CO3 separately, which form two types of new cement-soils. Comparing with the undisturbed soil and the 20% cement-soil results in that the alkalescence additive can contribute to generate a large number of needle-like, rod-like or fibriform hydrate calcium silicate crystals. The hydrate calcium silicate crystals can inhibit the generation of calcic alum crystal which would bring expansive action. At the mean time, it can separate out hydrate calcium. The hydrate calcium silicate crystals and hydrate calcium constitute the filling and inclusions together among soil particles and on the surface of soil particles, which cut down the interstices of cement soil obviously. Therefore, the soil mass can be reinforced and the intensity can be improved. Test results show that the soil-cement strength increases more than 20%. The composite foundation bearing capacity is not less than 120kPa.