Full aperture backscattering diagnosis system of Shenguang Ⅲ laser prototype facility is optimized under the condition of complex laser optical mirrors. Nonlinear scattering signals are collected by long focus Fresnel lens and focalize at the focus as background light is filtrated by vacuum spatial filter. Nonlinear scattering signals have been separated in space from background light. Then coaxial problem of nonlinear scattering signals with background light is resolved. Scattering light energy and spectrum of full aperture backscattering is measured in hohlraum experimental on Shenguang Ⅲ laser facility. Full aperture backscattering diagnosis system after optimization is validated by the hohlraum experiment, which offers possible condition and important referrence for exact laser-target coupling and energy balance precision diagnosis.