Marine engineering steel is the key material for the construction of major marine infrastructure projects. Due to the harsh environment in the deep sea, the mechanical properties such as strength, low temperature toughness and so on of the marine steel are required to be higher. In this work, the weldability of a Fe-Cr-Ni-Mo high-strength steel was studied, and the microstructure and impact toughness of the steel after welding thermal cycling at different peak temperatures were analyzed. The results show that the average impact toughness of characteristic heat affected zone under different temperatures increases first and then decreases with the increase of peak temperature (T_p). The microstructures of coarse grain heat-affected zone (CGHAZ, T_p=1320 ℃) and fine grain heat-affected zone (FGHAZ, T_p=1020 ℃) are quenched martensite. Because of the coarse grain size, the impact toughness of CGHAZ is poor, which is lower than that of FGHAZ. The microstructure of inter-critical heat-affected zone (ICHAZ, T_p=830 ℃ and T_p=760 ℃) is composed of quenched martensite and tempered martensite. Due to the randomness of the proportion of the interfaces between the mixed microstructures near the V-notch, the impact energy values of ICHAZ fluctuates greatly. The homogeneous fine grain structure in ICHAZ (T_p=830 ℃) has a crack arrest effect during the impact deformation, which makes the characteristic zone have the best impact toughness. Although the grain size in ICHAZ (T_p=760 ℃) is also fine, the existence of the ultra-fine grain zones (the grain size in which is only 1~2 μm) benefits the formation of secondary voids under the impact load. The undissolved M₂C and MC precipitations in matrix promote the connecting of secondary voids and then form the secondary cracks. As a result, the impact toughness of the characteristic zone is poor, and becomes the weak region of HAZ.