Abstract: In this paper, the mechanical equation that is suitable for the instrument system of water level observation is given The methods that determine the coefficient of the souation are discussed. On the basis of the discusion of the solutions of the equation, it is suggested tht the response of a water level instrument to fluctuation of the water evel in a wall depeale on the natural frequency of a instrument system, the natural frequency of a instrumen systen dePends on buoyancy coefficient of a buoy and effective mass, the buoyancy coefficielt depends on the radius of a buoy and a well caing. Furthermore, the buoyancy coefficient in very sensitive to the variation of a buoy radius. Based on calculation it is considered thatthe response of a water level instrument SW-40 to fluctuation of level in a wall is like that of seismograph whose natural period is several seconds.