Ocean engineering steel has been rapidly developed with the exploration of ocean resourses. A 690 MPa grade low carbon bainite steel was designed for ocean engineering, to upgrade performance by microstructure control and the refinement and dispersion control of precipitates. This steel was tempered on--line with rapid heating rate after control rolling and accelerated cooling process. The results show that the mechanical properties, especially the strength--toughness balance, are strongly influenced by the transformation of untransformed austenite and the condition of precipitates. When fast tempering at 550℃, microstructure recovered and few precipitates appeared, bringing strength down seriously, untransformed austenite turned into martensite/austenite (M/A) islands on cooling. When the tempering temperature reached 600℃, the size of the M/A islands transformed from untransformed austenite decreased slightly, Cu and Nb/Ti precipitates increased greatly, bringing an apparent improvement in strength. When tempering temperature reached 660℃, precipitates got coarsened and made the strength decreases, the untransformed austenite formed retained austenite film between the laths, improving toughness and making the best strength—toughness balance. When the tempering temperature reached the top at 700℃, the precipitates got further coarsened, the untransformed austenite on cooling turned into large M/A islands, bringing toughness and the strength—toughness balance down again. In general consideration, fast heating tempering at 600—660℃ could make the steel has the best strength—toughness balance.