Disclosed is a heat treatment process for lowering the yield ratio of ultra-high-strength marine steel. Critical temperature quenching and high temperature short time tempering are adopted to obtain the low-yield-ratio ultra-high-strength marine steel, that is to say, the quenching temperature is Ac3+/-20 DEG C, the heat preservation time for quenching is 1.4-1.8 min/mm, the tempering temperature is Acl-(80-150) DEG C, and the heat preservation time for tempering is 0.5-2.0 min/mm. According to the heat treatment process, under the circumstance that matching of the strength and toughness of a steel plate is ensured, the yield ratio of the ultra-high-strength marine steel is significantly lowered, thereby completely meeting the requirements of a ship structure, an ocean platform, etc. which need the ultra-high-strength marine steel of the 420 MPa-690 MPa level or above the 690 MPa level.