A computational fluid dynamics (CFD) simulation is performed to investigate the influence of lateral flow on the orifice flow in a trough-type liquid distributor. The discharge coefficients from the simulation are in good agreement with the experimental values, indicating that the CFD simulation is accurate in describing the outflow through orifices. The lateral flow near an orifice can change the velocity and pressure distributions of flow regions in front of this orifice, causing a decrease in the discharge coefficient. This phenomenon is supported by the theory of flow past a blunt body. An important implication derived from this finding is that the influence of lateral flow should be minimized in the design of a trough-type liquid distributor, because the decrease in the discharge coefficient leads to non-uniform outflow. The structure of a trough-type liquid distributor is optimized to improve the liquid distribution performance by reducing the influence of lateral flow.