Abstract: To study the sound insulation performance of composite plate structures under oblique incidence of sound waves, a method by deriving the transfer matrix of the multi-layer composite plate based on the bending wave theory was established to calculate the transmission loss. The transmission losses of single-layer and double-layer steel plates were calculated. The results are in good agreement with the published literature and experimental results, which validates the present method. Subsequently, the sound insulation performance of composite plate structures composed of simple plate, sound-absorbing material, and perforated plate was investigated. The results show that as the incidence angle increases, the through frequency shifts to higher frequency. Increasing the thickness of the sound-absorbing material can significantly improve the high-frequency sound insulation performance, but the low-frequency improvement is not obvious. The perforated plate can suppress the through frequency caused by resonance, but the transmission loss at high frequencies decreases. Sound insulation performance at high frequencies can be enhanced by adding an intermediate-layer plate.