Abstract: Mn-V low alloy steel is taken as the research object, and the fracture surface and microstructure of samples that are not qualified for impact properties after different heat treatments were analyzed.The results show that hot rolled widmanstatten structure and banded structure are the main reasons for unqualified impact toughness of materials.After normalizing, the widmanstatten structure and some banded structures are eliminated.However, due to the improper control of cooling rate, a large number of M/A island massive structures are produced and cleavage cracks occur at the interface between M/A island and ferrite, which destroy the continuity of the matrix and cause extremely low impact toughness.After quenching and tempering at 920 ℃ and tempering at 650 ℃, the banded structure almost disappears, mainly polygonal ferrite and tempered sorbite, with high toughness, which improves the impact toughness of the material.