The influence of addition of alloying elements of medium carbon Fe-Mn and Ni in the welding flux on the microstructure and toughness of the weld seam of FV520(B) steel was investigated by means of impact test, metallographic microscope, SEM and X-ray diffractometer. The results show that the microstructure of the weld seam of FV520(B) steel mainly composed of tempered sorbite and lath martensite with some residual austenite and secondary phases. With the increasing amount of alloying elements Mn and Ni, the microstructure of weld seam became finer, and the lamellae of martensite became thinner and distributed more uniformly. The addition of allying elements into a basic flux rather than an acidic ones showed much higher effectiveness in improvement of the toughness of the weld seam. The induced Mn and Ni can enhance the austenitic amount in the weld seam, which in turn plays an important role in enhancing its toughness. Moreover, the induced Ni rather than Mn was more effective in enhancing the toughness of the weld seam.