A highly efficient and ultra-compact broadband interlayer coupler based on a random grating structure has been developed using the particle swarm optimization (PSO) algorithm. This device consists of two vertically stacked waveguides with inverse structures and a random grating pattern. For the TE mode, the coupling length is as short as 4 μm, with a 200 nm interlayer gap, achieving demonstrating 93% coupling efficiency at a wavelength of 1550 nm. The coupling efficiency remains above 90% across a broad wavelength range from 1503 nm to 1600 nm. Even when there is a considerable deviation of over 100 nm between the waveguides, the efficiency still stays above 90%, demonstrating impressive robustness against process variations. The coupler designed with an inverse structure and measuring 4 μm in length outperforms the ordinary 10 μm long inverse taper waveguide, setting the stage for the advancement of miniaturized and efficient interlayer coupling devices in silicon-integrated photonic systems.