The marine pipeline is an important energy transportation platform today. It can cause underwater noise pollution due to sound radiation caused by pipeline vibration during oil and gas transportation. This study focuses on the noise-generation mechanisms of marine composite pipelines under internal and external flow excitation. We investigated the impact of different materials, diameters, and wall thicknesses on the sound radiation characteristics of the pipeline under internal flow excitation. The relationship between radiated sound power and flow velocity was analysed. Additionally, we discussed the radiated sound field of marine composite pipelines under external flow excitation. Our results show that the radiated sound power under internal flow excitation is over 20 dB higher than that under external flow excitation in the 10 Hz to 2000 Hz frequency range. We conducted an experimental test near Zhoushan city, China, where the internal flow was driven by a pump and the external flow represented ocean currents. The results confirmed that the main noise-generating mechanism of marine composite pipelines is internal flow excitation. The numerical findings were validated by the experimental test. Overall, this paper provides a foundation for future investigations into the sound radiation of marine composite pipelines.