Considering the non-equal strain between the column and the surrounding soil within the composite ground with undrained columns under flexible foundation, the governing differential equation of the average excess pore water pressure in the soil around columns and the corresponding solution conditions under time-dependent load were derived. Based on the one-dimensional consolidation theory of natural soil foundation under time-dependent load, the corresponding analytical solutions were developed by the function transformation method. The average excess pore water pressures in the composite ground and the surrounding soil, and the overall average consolidation degree of the composite ground were included in the analytical solutions. The proposed solution was compared with the existing analytical solutions in the literatures and the numerical result obtained from the finite element method (FEM) to validate its accuracy. The results show that the proposed solution and FEM results have a good match, and the accuracy of proposed solution is higher than those of the existing consolidation models. The consolidation rate of the composite ground with undrained columns under embankment is overestimated by the analytical consolidation solutions based on the equal vertical strain assumption.