Objective: Traditional methods for floating and transporting immersed tunnel elements at sea often involve the use of tugboats for towing. This approach results in the vessel and the tunnel element moving independently, making it difficult to control the attitude of the immersed tube and leading to low navigation speeds. The Shenzhen-Zhongshan Bridge project in China, however, utilized an integrated vessel for the transportation and installation of immersed tubes. This specialized construction boat combines the operations of floating, positioning, immersion, and installation of tunnel elements. The integrated vessel measures 190.40 m in length, 75.00 m in beam, 14.70 m in depth, and 23 200 t in weight. It is equipped with two main propulsion systems, each capable of delivering 9 280 kW and eight side thrusters ranging from 2 600 to 3 000 kW. The integrated vessel, connected rigidly to the immersed tube through supports and cables, demonstrated rapid floating capabilities in the Shenzhen-Zhongshan Bridge project, achieving a maximum navigation speed of 5.8 kn and covering a 47.0 km floating route in just 7-8 h. While this high-speed floating navigation enhances operational efficiency, the safety of both the vessel and the transported elements during floating remains a significant concern. A notable issue observed is the synchronization of the attitude between the element and the vessel. During acceleration, a relatively significant longitudinal tilt occurs, necessitating in-depth analysis to understand the hydrodynamic mechanisms behind this trim occurrence during high-speed floating of oversized immersed tubes, as well as to assess the impact of sustained trim on the safety of floating navigation and the loss of propulsion efficiency for the vessel. Methods: This paper presents a theoretical analysis comparing the resistance distributions of immersed tunnel elements in calm water with those under navigation at specific speeds. In situ measurements were conducted to observe attitude changes during the floating process. A numerical model describing the floating condition of a single tube element was developed using FLOW-3D software to analyze the resistance distributions and attitude changes at approximately 4.0 kn. Additionally, a comprehensive numerical model of the vessel-tube connection was established using computational fluid dynamics methods, with a scaling ratio of 1∶40 for model-scale simulations. These models simulated the flow field changes around the integrated vessel and the immersed tube at navigation speeds of 4.0 and 6.0 kn. Results: Through theoretical analysis, in situ measurements, and numerical simulations, the following conclusions were drawn: (1) The geometric shape of the immersed tube, which was a nonstreamlined rectangular box, resulted in significantly greater end face (bow face) resistance than streamlined vessels. This end face resistance was the main component of the navigation resistance for the immersed tube. (2) At certain navigation speeds, a downward flow field formed by the water at the bottom of the bow end was identified as the primary cause of the bow-down tilt of the immersed tube. This vertical flow field decreased the water pressure in the area near the bow end, leading to a significant trim phenomenon. (3) The total frictional resistance caused by the viscosity of water was found to be only approximately 1.50% of the total resistance, making its impact almost negligible. Conclusions: Measurements of the integrated vessel's attitude during the rapid floating of immersed tubes indicate a significant longitudinal tilt. A relationship between the trim angle and navigation speed is established through these measurements. By combining numerical and theoretical analysis methods, it is possible to analyze the state of the flow field around the immersed tube under high-speed floating conditions. The analysis suggests that the longitudinal tilt of the immersed tube is related to the flow field formed at the bow of the immersed tube, which reduces the dynamic pressure at the bottom of the bow end. This reduction in pressure generates a rotational moment in the bow tilting of the immersed tube.