To find out the rectifying performance of the orifice plate in a gas-liquid separator, the three-dimensional flow field of the gas-liquid separator is numerically simulated, usingk-ε model and DPM model. By comparing the velocity distribution and separation efficiency of the separator without orifice plate and with different orifice baffles which have pore diameters of 16~24 mm and porosity factors of 25.56%~31.80%, the rectifying performance of orifice plate is obtained. The results show that, orifice plate can rectify the flow field, reduce the inhomogeneity of velocity, and inhibit backflow. It makes flow field more well-distributed and steady, which is good for sedimentation separation. The pore diameter and porosity factor of the plate both should not be too large or too small, and the plate with 20 mm pore diameter and 31.8% porosity factor has a better performance in this study. The results can provide guidance for the structure design of gas-liquid separator and the type selection of the orifice plate.