Ultra-compact multichannel optical waveguide crossings are designed by a particle swarm optimized inverse-design method. The 2× 2 TE (TM) device has an ultra-small footprint of 2× 2 μ m 2 and exhibits a high transmission efficiency of 69.1%(89.7%) and low crosstalk of− 34.4 dB (− 84.8 dB) at 1550 nm. The devices can work within a large range of pixel size and waveguide width, suggesting high process tolerance of the design method. To further extend the functionality, a 3× 3 device with the same size is designed, which exhibits a remarkable transmission efficiency of 47.6%, 61.8%, 67.2%, and low crosstalk of− 33.3 dB,− 39.2 dB, and− 49.9 dB, respectively, for the three transmission paths. In addition, by adjusting the structure, other types of 3× 3 devices with different transmission paths are demonstrated. The ultra-compact broad-bandwidth high-process-tolerance optical waveguide crossings have great potential in high-integration photonic-integrated circuits.