With the increasing demand and exploitation depth for offshore oil and gas, offshore platforms are becoming larger and the performance requirements and size for offshore platform of ultra-heavy plates are also increasing. Due to the large plate thickness and the limitation of manufacturing techniques, inhomogeneous microstructures and mechanical properties along thickness direction are great challenges for offshore platform of ultra-heavy plates. In this work, variation of microstructure and its effect on mechanical properties for the 117 mm-thick Ni-Cr-Mo-B industrial ultra-heavy plate were investigated by means of OM, SEM, TEM and EBSD observation, in combination with the tensile and impact toughness test. The results show that yield strength reduces gradually from the surface (798 MPa) to the center (718 MPa) and elongation almost keeps constant around 20.0%~22.0% for the 117 mm-thick plate. It is noted that impact energy at -60 ℃ increases first from 35 J at the surface and reaches its peak 160 J at the depth of 1/8T (T—thickness of plate), and then drops to the minimum about 20 J at the center, which suggests that impact energy curve along the whole section varies sharply and exhibits like letter 'M'. Lath width, boundary carbide size and intragranular carbide size are all gradually increasing from the surface to the center, i.e., from 198.7 nm to 500.6 nm, 130.6 nm to 226.6 nm, 45.8 nm to 106.2 nm, respectively, and there are also some blocky areas at the center, all those indicate that refinement strengthening and precipitation strengthening would decrease, as well as the gradual decrease of yield strength. Also, from the surface to the center, effective grain size (EGS) decreases first and then increases. The surface and the center have larger EGS (2.2 μm and 2.7 μm, respectively), which indicates that they have weaker resistance to cleavage crack and exhibit lower impact energy. However, the 1/8T position has smaller EGS (1.7 μm) while obtains higher impact energy.