Abstract: 690 MPa grade high strength low alloy (HSLA) steel of 27 mm in thickness was welded by self-developed electrode of 4.0 mm in diameter without preheating. The effects of heat input on microstructure and mechanical properties of weld metal were studied and the mechanism of strength-toughness was revealed. The results show that heat input had significant effect on microstructure and mechanical properties of weld metal, which increased from 13 kJ/cm to 19 kJ/cm. With the increasing of heat input, tensile strength and hardness of butt joint decreased, −50 ℃ impact energy firstly increased and then decreased. Good strength-toughness was obtained under the heat input condition of 16 kJ/cm. The tensile strength of butt joint was 828 MPa, −50 ℃ impact energy of weld metal was 71 ~ 90 J, the average value was 80 J. The lower strength-toughness properties of weld metal under 13 kJ/cm and 19 kJ/cm heat input conditions were related with the formation of lath bainite and ferrite side plates, the formation of coarse M-A, respectively. Under the 16 kJ/cm heat input condition, good plasticity acicular ferrite was sufficiently formed and acicular ferrite and bainite were distributed in an interwoven pattern. Meanwhile, small and dispersively distributed M-A had no obviously negative effect on toughness of weld metal. Therefore, weld metal with excellent strength-toughness had been obtained.