The vibration characteristics of a submerged rotary double-shell is analyzed using rotary shell element and fluid element. The coupling between the double-shell system and acoustic medium is taken into account by fluid finite elements for the fluid field between the inner and outer shells, and by the boundary elements for the fluid field outside the shell. The energy functional of the structural system is worked out by D'Alembert principle. A frequency e-quation for the motion system is obtained by the Novozhilov's shell theory and the Hamilton variational principle. Fair agreement is seen between the measured and predicted natural frequencies of the rotary double-shell system both in air and in water.