The inverse acoustic scattering of point sources by an impenetrable cavity is considered. The scattered fields incited by point source waves are measured on a closed curve inside the cavity. We prove the validity of the factorization method for reconstructing the shape of the cavity. Two explicit examples for circular cavities are given to show the feasibility and effectiveness of the factorization method. Finally, we present some numerical examples in 2D. The reconstructions are as satisfactory as the exterior scattering problems and change only slightly for different measurement curves, different boundary conditions and different wave numbers.