Based on classical laminated plate theory and taking the stiffener bending and torsional movement into account, the sound transmission theoretical model of an infinite single stiffened laminated plate excited by a plane sound wave is developed. Transmission Loss (TL) formula is derived by using space harmonic expansion and the principle of virtual work. Then, the accuracy and effectiveness of the proposed model is qualified by comparing model predictions with calculation results in the literature. Finally, the impact of different parameters on the sound insulation performance is studied through numerical calculation. The results indicate that the increasing laying angle has no benefits for the sound insulation performance. Material loss factor only affects the sound insulation at structural resonance frequency. Within the low frequency range, the sound insulation performance decreases significantly as the stiffener spacing is increased. But, in the whole frequency range, the sound insulation performance is improved. The angle of incident wave plays an important role in sound insulation. The oblique incident sound wave transmits through the structure more easily than that of the normal incident sound.