Influence of cavity wall elasticity on resonant frequency of small underwater cylindrical Helmholtz resonator is studied theoretically and experimental. Based on the theory of electroacoustic analogy, the simplified low frequency lumpedparameter model of the Helmholtz resonator is constructed. A general, convenient for calculation expression of the resonant frequency is given by circuit analysis. The influence of the thickness and the material of the resonator on the resonant frequency is investigated by numerical method. And the approximate rigid conditions for small underwater cylindrical Helmholtz resonators of different sizes are given. Small cylindrical Helmholtz resonators of different wall thickness and material were tested in a standingwave tube filled with water. Experimental results well testified the theoreticl results and the approximate rigid condition. This paper is useful for the design and application of the underwater cylindrical Helmholtz resonators.