The foundation style of such construction as liquefied oil storage tank and environmental protected reaction pool is mostly annular foundation. In the paper, the annular foundation will be put on WINKER foundation, and then the annular foundation will be simplified as a curve beam, the cubic B spline functions as displacement functions of curve beam. The dynamical equation and the nature frequency of annular foundation based on the instantaneous principle of minimum potential energy, and put forward Spline Function Method to solve the nature frequency of annular foundation. Numerical examples are provided to demonstrate the simplicity and effectiveness of the present method. The results are in good agreement with experimental data.