When studying the sound insulation design of composite aircraft fuselage, it is usually assumed that the thermal/acoustic insulation blanket (TAIB) made of sound absorbing materials covers the entire area of the fuselage. However, due to the existence of pipelines, cables and equipment, the TAIB could be partially thinned. This paper aims to study the effect of the thinned TAIB on the sound insulation properties of the composite aircraft structure. The simulation model of the composite fuselage panel composed of composite skin, stringers, frames and TAIB is established by using statistical energy analysis method, and validated by test results. The influence of the distribution of TAIB on the sound insulation of composite panel under space constraints is studied. The results show that the influence of the TAIB thickness reduction on the sound insulation of the composite panel is mainly in the middle and high frequency bands above 400 Hz, and the decrease of the sound insulation is positively correlated with the reduction of the thickness of the TAIB. Keeping the TAIB thickness over 3 inches can maintain the sound insulation decrease within 3 dB. When the TAIB thickness is lower than 3 inches, adding a rubber layer on the side of the trim panel can significantly improve the sound insulation performance of the composite panel in middle and high frequency bands from 800 to 2000 Hz.