Abstract: In order to evaluate the effect of the complicated working conditions in small diameter pipeline on the passing capacity of all-polyurethane pig, the possible causes for the whole fracture of all-polyurethane pig were analyzed. 3 D explicit dynamics simulation analysis models for all-polyurethane pig with diameter of 323.9 mm passing through a straight pipeline with single-sided deformation and scaled pipeline were established respectively with the Scan To 3 D technology and the mechanical property test of polyurethane material based on the actually-deformed pipeline with diameter of 323.9 mm on site. Through simulation model solution and result analysis, it is found that the all-polyurethane pig has high passing capacity and good pigging effect under the conditions of single-sided dent deformation and internal wall scaling of straight pipeline. The bending stress and contact stress generated when the all-polyurethane pig passing through a single-sided dent deformation of pipeline are not sufficient to cause serious scratches on the surface, cup tearing or whole fracture of the polyurethane pig. In the process of pig passing through the scaled section of the pipeline, large axial compression deformation is generated on the barrel between the front and rear cups, and serious radial outward expansion and tearing is the main reason for the whole tearing fracture of the pig barrel. Generally, the above research conclusions have profound practical significance for improving the schematic design level and promoting the engineering application of small-diameter all-polyurethane pig in China.