Abstract: In the paper,the probability analysis method of the bearing capacity of cement deep mixing composite foundation is investigated,which will push the developing of the reliability design method of composite foundation.The bearing capacity of cement deep mixing composite foundation is calculated using the area ratio formula,and the model uncertainty factor is also investigated through some case analysis.First,the mean and variance of the bearing capacities of the cement pile and the soil between the cement piles are ascertained after analysis,and so the probability charcteristic of the composite foundation is deduced.Then more reliable probabilistic characteristic of the bearing capacity is gained using Bayes’s theorem after considering the results of the plate-loading test.Finally an example is used to illustrate the calculating process.