The cement soil pile method using fly ash instead of some cement as a cementitious material has been gradually applied in the reinforcement treatment of weak foundations in recent years. Aiming at solving the problem of soft foundation of the Dongting Lake area, the unconfined compressive strength, water content and pH value of soil samples in the central part of the cement-soil specimens with different fly ash contents were tested indoors, by combining the foundation treatment project of a floodgate in the area. And the combined exponential model of the unconfined compressive strength of cement soil with different fly ash content was established. The results showed that the unconfined compressive strength of cement-soil specimens decreased with the increase of fly ash before 60 day age, but the strength values of four specimens are close at 90 day age. The moisture content of the part increased with the increase of fly ash content after 28 day age. the pH value of the soil sample solution in the central part of the cement soil test specimen always decreased with the increase of fly ash content. The ash content cement soil pile scheme is compared and selected. The combined exponential strength model can better describe the influence of fly ash content and age on the unconfined compressive strength growth law of cement soil. Through the comparison of different cement ash mixed cement soil pile schemes, the amount of fly ash in the cement soil of Dongting Lake area should be about 20%.