Abstract: This paper discusses the free vibration governing equations of a coupled systemwith the emphasis on the calculation of dynamic water pressure vectors and the added mass matrix. It also makes a brief introduction about the solution of the dynamic behavior of structures under water medium. The calculation results obtained from a practical example are presented, which showthat the basic frequency of the turbine obtained with the structure-liquid coupling effect taken into consideration is less than that without the coupling effect, and that all the modes of vibration are obviously different from each other.