Optical materials with centrosymmetry, such as silicon and germanium, are unfortunately absent of second‐order nonlinear optical responses, hindering their developments in efficient nonlinear optical devices. Here, a design with an array of slotted nanocubes is proposed to realize remarkable second harmonic generation (SHG) from the centrosymmetric silicon, which takes advantage of enlarged surface second‐order nonlinearity, strengthened electric field over the surface of the air‐slot, as well as the resonance enhancement by the bound states in the continuum. Compared with that from the array of silicon nanocubes without air‐slots, SHG from the slotted nanocube array is improved by more than two orders of magnitude. The experimentally measured SHG efficiency of the silicon slotted nanocube array is high as 1.8 × 10−4 W−1, which is expected to be further engineered by modifying the air‐slot geometries. The result can provide a new strategy to expand nonlinear optical effects and devices of centrosymmetric materials. An array of slotted nanocubes is proposed to achieve strong second harmonic generation (SHG) effects in Si. Bound states in the continuum‐induced electric field enhancement and slot‐induced surface effect play a crucial role in the SHG. The high conversion efficiency of 1.8 × 10−4 W−1 can be experimentally obtained. This strategy can boost efficient SHG even in centrosymmetric materials.