Abstract: The effects of plastic deformation on the microstructure evolution and toughness properties of X100 pipeline structure steel were studied by scanning electron microscope (SEM), transmission electron microscope (TEM) and mechanical test. The results show that both acicular ferrite and granular bainite change remarkably with the value of stress under the same aging temperature and time. Simultaneously, the high density dislocation agglomerates around the M/A island, forms the subgrain boundary and the crystal boundary. After the deformation, the grain sizes decrease, while there are many subgrain structures distributed in crystal, and the material impact toughness drops remarkably due to M/A constitution distributed on the crystal boundary and the deformation flaws. Correspondingly, the sensitive coefficient of strainaging of grade X100 pipeline meets the needs when the temperature is above -34 °C.