Abstract: To ameliorate the low-frequency sound absorption performance of bulk porous materials， a theoretical model for predicting the sound absorption of the composite structure incorporating flexible ultra-thin and bulk porous materials is derived. The model integrates Johnson-Champoux-Allard （JCA） model and acoustic impedance model for flexible ultra-thin materials， employing non-woven fabric and melamine cotton as illustrative instances. The accuracy of this model is validated through experimental verification， and particle swarm optimization （PSO） is employed to optimize the parameters of the composite structure. The analysis results indicate that the addition of a layer of non-woven fabric on the surface of traditional bulk porous materials significantly enhances the low-frequency sound absorption performance. This paper provides a theoretical foundation for the determination， analysis and optimization of the sound absorption performance of the composite sound-absorbing structure composed of flexible ultra-thin and bulk porous materials， Additionally， it presents an effective method for improving the low frequency sound absorption performance without changing the thickness of the original sound absorber.