In this study, the effects of welding speed on the microstructure, microhardness, and mechanical properties of laser welded 1700MS ultra-high strength steels were investigated. Experimental results suggest that the welding process involved martensite tempering transformation, leading to a large number of granular tempered martensite in the sub-critical heat-affected zone (SCHAZ). The size and quantity reduced significantly as the distance from the fusion zone's center (FZ) increased. Due to the tempered martensite, the welded joint had a serious decrement in hardness. With a softening degree of 52.71%, the maximum and minimum hardnesses were 609 HV and 321 HV, respectively. Because of the softening, the structure became uneven with a soft zone (heat-affected zone) in the center and hard zones (FZ and base metal) on the sides. This could cause a decrement in the mechanical properties of the welded joint during the stress process. As a result, increasing the welding speed can reduce the tempering degree and width of SCHAZ, thus improving the mechanical properties of the joints.