Effect of quenching processes on the mechanical property and microstructure of a newly designed ultra-high strength marine engineering steel of low carbon (C<0.05) NiCrMo was investigated by means of thermo-Calc software, optical microscopy, scanning and transmission electron microscopy. Results show that secondary hardening occurred for the steel quenched from 910℃ and then aged at 525℃, resulting in a maximum peak hardness was 369 HV, while secondary martensite microstructures emerged for the steel quenched from 910℃ and then aged at 700℃ by air cooling, resulting in a peak hardness 361 HV. Thermo-Calc calculation result revealed that the mean particle radius of (Nb, Ti)C was obviously reduced with the decreasing quenching temperature within the range of 820~910℃, and the refined (Nb, Ti) C particles could effectively suppress the growth of austenite grains, thus improving grain boundary density of high or low angle in the matrix, which led to the increment of strength and toughness. Among others, the steel quenched from 820℃ presents the highest strength up to 1084 MPa, impact energy of 76 J for V-type impact test at -80℃, and the fracture fiber rate was up to 100%. Fractograph- and crack propagation-observation showed that the refinement of microstructure and second phase could hinder the expansion and fracture of dimples, while the refined martensite packet and block could significantly alter the crack propagation direction. Finally, the steel quenched from 820℃ presents the maximum unit length of 15 μm for the crack propagation path, implying a high toughness of the steel.