Underwater data centers are important information processing hubs in ocean strategy. A new type of water-cooled backboard system that fully utilizes natural cooling sources and operates with no host and low energy consumption was developed to meet the high heat density and energy efficiency requirements of underwater data centers, and a 3.5:1 scaled test platform was built for underwater testing. The results show that when the ambient water temperature is 25 ℃ and the load is 2.4 kW and 3.6 kW, the inlet/outlet air temperature of the underwater data center server is less than 45 ℃/55 ℃, and the cooling effect of the system is good. In order to deduce the operational performance of full-scale engineering prototypes in actual environments, a one-dimensional steady-state heat transfer model was established. The simulation results show that the system PUE is below 1.075 when the power is 50-70 kW and the underwater ambient temperature is 25 ℃. The study also analyzed the applicability of the new water-cooled backplate system in underwater data centers in different sea areas. When the system server power is 70 kW and the ambient water temperature is 5-25 ℃, the system PUE is 1.051-1.068.