Abstract: In order to solve the problem of uneven toughness of heat-treated X80 steel, fracture analysis, microstructure analysis and backscattered electron diffraction analysis of typical impact samples were carried out. The results show that the non-uniformity of microstructure and the existence of large massive quasi-polygonal ferrite lead to the low content of large grain boundaries, large effective grain size and poor uniformity, which are the fundamental reasons for the cleavage brittle fracture of the impact sample of the heat-treated X80 steel. To obtain fine quasi-polygonal ferrite can increase the relative content of large grain boundary in X80, refine the effective grain size, and guarantee the strength and toughness of heat-treated X80 steel.