Abstract: Q355E novel low-alloy high-strength steel is taken as the research object to investigate the microscale mechanism of microstructure evolution of low-alloy high-strength steel during heat treatment. Scanning electron microscope (SEM),transmission electron microscope(TEM), and X-ray diffraction(XRD) are employed to characterize the microstructural morphology and evolution of the tested steel before and after lanthanum treatment via the addition of La-Fe master alloy under different heat treatment process parameters. The experimental results show that the solution temperature exerts a significant effect on the austenite grain size and martensite lath size, and the optimal solution temperature range is determined to be 800-900 ℃. In the tempering process, the austenite content remains stable with increased carbon content at 300-320 ℃, while the austenite content decreases when the tempering temperature exceeds 320 ℃, confirming 320 ℃ as the optimal tempering temperature. Cryogenic treatment at -40 ℃ achieves the optimal matching state of austenite content and carbon content. Moreover, the addition of La-Fe master alloy promotes the transformation of internal inclusions into spherical rare-earth composite oxysulfides, which effectively improves the comprehensive properties of the steel material.