Based on computational fluid dynamic (CFD) method, the simulation of a normal vertical-vessel type centrifugal gas-liquid separator was carried out, by using Gambit and FLUENT soft packs. It is found that the axial symmetry of the flow field is bad, the gas-content near underflow outlet is higher, which indicates the separating effect of the separator is not satisfied. Through structural optimization, including changing the underflow outlet from side to bottom, designing double inlets, reducing the vessel diameter, extending the overflow tube to the bottom of inlet tube, changing the bottom part of the separator from cylindrical to conical structure, etc., so as to improve the axial symmetry of the separator flow field, including gas phase distribution, then to enhance its separation effect. CFD analysis result of velocity distribution is in accordance with Laser Doppler Anemometer (LDA) result qualitatively. All of these will provide some reference on structural optimization. It is concluded that CFD aided optimization design is very helpful for fluid machinery design, which can guide experimental work and decrease the experimental time and cost to a great extent.