Abstract: To determine impacts of structures and sizes of pipeline pig to flow fields inside pipelines during pigging operations, and to determine optimal sizes and structures of pipeline pigs, take straight-plate pipeline pig with adjustable velocities as an example, working models for pipeline pigs with adjustable velocities and three different length/diameter ratios have been established for pipeline pigs by using Gambit (a FLUENT pre-processing software). In addition to division of grids, FLUENT fluid analyzing software has been used to perform numerical simulation to flow fluids in pipelines with different length/diameter ratios during pigging operations. As results, pressures, velocities and distribution of turbulent kinetic energy can be determined for pigging operations in pipelines with length/diameter ratios. With the length of the pipeline pig as the objective for optimization, results of numerical simulation have been reviewed and analyzed to reach following conclusions: flow fields in vicinity of pipeline pig are characterized by violent fluctuations and dramatic increase of turbulent kinetic energy and intensity. In addition, they displayed maximum values in internal flow fields of pipeline pigs; intensities of turbulent flows in fields downstream of the pipeline pig reduced to certain degrees, but they still can be classified as flow with high turbulent flow intensities. Relevant results show that the fluctuations of various indexes are low at the ratio of L/D=1. It can be seen that this ratio may have minimum impacts to flow fields downstream. Such conclusions coincided well with actual conditions. Relevant researches may provide theoretical supports for design optimization of pipeline pig.