Abstract: The influence of silicon (Si) on the microstructure and toughness of 800 MPa grade high-strength low-alloy (HSLA) deposited metals was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). Experimental findings revealed that the yield strength of the deposited metal (0.035C-0.45Si-1.47Mn-2.56Ni-0.68Cr-0.62Mo) increased from 850 MPa to 895 MPa, and the tensile strength rose from 917 MPa to 954 MPa, while the impact energy absorption at −50 ℃ decreased from 115 J to 73 J, as the Si content increased from 0.45% to 0.66%. The microstructure of the deposited metal with 0.45% Si primarily comprised lath bainite, with a smaller proportion of granular bainite and lath martensite. However, with an increase in Si content to 0.66%, the microstructure predominantly featured slender lath-shaped martensite, accompanied by some lath bainite. This increase in Si content from 0.45% to 0.66% led to a reduction in the transformation temperature of austenite → bainite/martensite mixed structure. As the Si content increased, the lath substructure and block substructure changed from interlaced distribution to parallel distribution, and both become slender. However, the size of packet substructure increased significantly, which reduces the proportion of the large angle grain boundary of the deposited metal, leading to a decrease in its impact toughness.