The principle of detecting submarine objects based on stimulated Brillouin scattering (SBS) is proposed. It is based on signal disappearance to determine the position of submarine objects, and temporal difference of pump signal and echo signal to calculate the distance submarine object. When laser is focused into the water, SBS occurs as the laser intensity is over the SBS threshold. On the other hand, SBS does not occur when some objects appear before the focus. Theoretically, within diffraction limitation, it can detect objects farther than 100 m with pulse energy of 1 J, pulse width 10 ns and emission diameter 1 m; experimentally, the measurment error is less than 10 cm within 2 m, which is determined by the pulse width and does not change with the detecting distance.