Spectra and influences of Rayleigh backscattering (RB) and stimulated Brillouin scattering (SBS) in the fiber-optic distributed disturbance sensor (FDDS) were investigated. Models of RB, double RB (DRB) and SBS in long fibers were established. By numerical simulation, it was found that optical signal-to-noise ratio is extraordinarily reduced due to SBS and RB, which results in location errors. Numerical results were confirmed by experiments and helpful to improving the location precision for applications with long monitored length.