Abstract: This study focuses on Ni-saving 5.5Ni steel for LNG storage and transportation and aims to clarify the effect of tempering temperature during the QLT (quenching-lamellarizing-tempering) process on its microstructure and mechanical properties. The correlations among lamellar microstructural features, reversed-transformation austenite, and crack propagation behavior were systematically investigated at different tempering temperatures. Combined with tensile tests, impact tests and characterizations including SEM, EBSD and XRD, the lamellar characteristics of microstructure, dislocation density, Taylor factor, texture evolution and reversed-transformation austenite were quantitatively analyzed. The results show that after tempering at various temperatures, the 5.5Ni steel exhibits a microstructure with alternating distribution of lath martensite and quasi-polygonal ferrite. As the tempering temperature decreases, the strength of the tested steel remains essentially stable, while the longitudinal impact toughness decreases. The elongation is influenced by the content and stability of reversed-transformation austenite, which initially increases and then decreases. Since this study employed longitudinal cryogenic toughness as the assessment criterion, the optimal tempering temperature was determined to be 610 ℃. At this temperature, the tested steel achieves the optimal balance of strength, ductility and toughness, with a yield strength of 670.70 MPa, a tensile strength of 754.40 MPa, an elongation of 20.40%, and a longitudinal impact energy of 145.97 J at -196 ℃. Higher tempering temperatures promote the formation of a distinct martensite/ferrite lamellar structure. While imparting excellent longitudinal cryogenic toughness to the material, this structure also introduces structural anisotropy, thereby reducing the transverse impact energy. During longitudinal impact, back stresses and strain gradients generated at the heterogeneous interface effectively deflect the main crack path, and the highly stable reversed-transformation austenite effectively blunts the crack tip. During transverse impact, the main crack tends to propagate along parallel lamellar interfaces, which weakens the crack resistance and leads to the reduction of transverse impact energy. In summary, this study provides a theoretical basis for microstructure control and heat treatment process optimization of low-Ni steels.