Nonlinear optical properties of stimulated Brillouin scattering (SBS) to signal detection in water are analyzed. With the threshold characteristics, SBS only occurs when the high power laser is focused in the SBS cell. When there is an object present in front of the focus, it leads to lower incident intensity and then SBS does not occur. The backward SBS signal depends on the focusing location. The nonlinear optical properties of SBS process in the focusing regime are analyzed theoretically. With the object coming near to the focusing center, the backward Stokes signal rises up from zero to a maximum, and then grows to saturation. The delay time of the echo signal to pump signal can give the object location. In experiment, the peak position of varying rate of energy can give object location.