We study an inverse problem of reconstructing a 3D object in an ocean waveguide from far-field measurements of scattered field.An indicator sampling method for fast imaging of scattering object is presented.The method does not depend on any acoustic and geometric properties of the scattering object.It is valid for obtaining good imaging of location and shape of the scattering object,even as far-field data are measured in a limited aperture and corrupted by noise.Generally,multiple scattering between object and waveguide boundaries has positive influence on reconstruction of the scattering object.