Abstract: LNG cargo tanks require safe and stable service at an ultra-low temperature of -162 ℃, which puts forward extremely stringent requirements for the strength and cryogenic toughness of structural materials. However, the widely applied 9Ni steel currently suffers from increased cost and welding magnetic arc blow defects due to its high nickel content. To reduce the nickel content of 9Ni steel for LNG carriers, cut down production cost and lower the dependence on high-nickel materials, this work adopted the TMCP-LT (thermo-mechanical controlled process+lamellarizing+tempering) process. The effects of lamellarizing temperatures ranging from 650 ℃ to 720 ℃ and tempering temperatures ranging from 500 ℃ to 600 ℃ on the microstructure, room-temperature tensile properties, and -196 ℃ impact absorbed energy of the 5.5Ni-0.25Mo tested steel were systematically investigated, and the corresponding strengthening and toughening mechanism was clarified. The results of multi-scale characterizations including OM, SEM, XRD and TEM, as well as room-temperature tensile tests and Charpy impact tests show that when the lamellarizing temperature increases from 650 ℃ to 700 ℃, clear interfaces are observed, grains are gradually refined, the volume fractions of intercritical ferrite and retained austenite decrease, the content of tempered martensite increases, and the strength of the steel decreases. A large number of fine blocky tempered martensite structures form and retained austenite disappears completely at a relatively high lamellarizing temperature of 720 ℃. With the increase of tempering temperature, the volume fraction of retained austenite rises, the dislocation density decreases, and the strength of the tested steel is reduced. The -196 ℃ impact absorbed energy of the tested steel under different processes is far higher than the minimum requirement of 100 J specified in the national standard GB/T 713.4—2023. The excellent cryogenic toughness is attributed to the matrix structure formed by high-degree recovery and the extremely low retained austenite volume fraction with a minimum value of 2.7%. The optimal TMCP-LT process is determined as lamellarizing at 680 ℃ combined with tempering at 500 ℃ to 530 ℃. The tested steel exhibits a yield strength of 610-632 MPa, a tensile strength of 696-721 MPa, a total elongation of higher than 19.5%, and a -196 ℃ impact absorbed energy of 183-219 J. All the performance indicators meet the requirements for 9Ni steel specified in GB/T 713.4—2023, which provides a theoretical basis for the industrial production of Ni-saving cryogenic steel for LNG carriers.