Abstract: To determine the separation performance of inlet pre-separated elements of underwater separator under high gas/liquid ratio, numerical simulation is performed with FLUENT, a hydrodynamic software. With consideration to three length/diameter ratios and three inlet inclinations of underwater separator, the physical models for 9 inlet pre-separated elementsare established. Through simulation, the internal velocity distribution cloud maps, velocity curves and fluid drop separation efficiency maps for all models are obtained and compared. It is concluded that internal flow fields in inlet pre-separatedelements are differently affected at various length/diameter ratios and inlet inclinations. In particular, at length/diameter ratioof 4:1 and inlet inclination of 0°, the most steady internal flow field and the most desirable gas/liquid separation performanceare obtained.