The separation of liquid-gas flows is essential for many industrial processes. Computational fluid dynamics including the algebraic slip mixture model and the Reynolds stress model is firstly adopted to analyze the impact of the number of inlets and the column diameter ratio on the degassing performance of a hydrocyclone. For separating the micrometer-sized bubbles, the structures with one to six inlets and seven different column diameter ratios are simulated. The results show that multiport designs are more suitable for liquid-gas separation, particularly for separation of small bubbles. However, as the number of inlets increases, the growth trend of separation efficiency is slowed. A longer column section is beneficial to small bubbles, but results in a greater loss of fluid energy.