Ocean current energy is the kinetic energy of seawater flow and is a low-cost, low-carbon, clean, and renewable energy source. Ocean current energy is mainly captured by the Ocean Current Turbine (OCT). However, as the depth of the water increases, the flow rate of the water decreases significantly. The main purpose of this paper is to study the low efficiency of current turbine capture due to low current velocities, with the main objective of achieving the effect of gathering and increasing the speed of low current energy. Firstly, this paper investigates the mechanism and mathematical model of the concentrated and accelerated flow of the diffuser shroud. Secondly, the velocity and pressure flow fields of the basic diffuser shroud are simulated and analyzed under different low flow velocity conditions (flow velocity below 1.0 m/s), different aspect ratios, and different diffusion opening angles. This paper further investigates the effect of speed increase for four different airfoil sections of diffuser shroud. Finally, measurements and verification are carried out with Particle Image Velocimetry (PIV) circular water hole experiments. The experimental results demonstrate the concentrated and accelerated results of the diffuser shroud. The basic diffuser shroud has a 39% increase in flow field velocity. The airfoil section diffuser shroud has a better velocity increase on the flow field than the basic diffuser shroud. The flow field flow rate increased to 58%. The optimum location for the speed increase is near the inlet and less than 0.5 times the inlet diameter. Compared to the simulation results, the experimental results of the PIV circulating water hole show an average error of less than 4%. The results of this paper provide a research basis for the efficient capture of current energy in deep-sea low-flow waters and a theoretical basis for the design of low-flow current energy turbines.