AlN crystal is a typical brittle and difficult-to-process material. To prepare AlN crystal substrates with a high efficiency and a high quality, slurries with different chemical compositions were designed to lap AlN crystals, and the mechanism of chemical action-assisted material removal was investigated by laser confocal microscopy and X-ray photoelectron spectroscopy. The results show that the alkaline slurry is beneficial to obtaining the optimum lapping results, the material removal rate is 23 μm/h, and the surface roughness (Ra) is 122 nm. The diamond abrasive particles in the slurry have a good dispersibility in alkaline environment. The alkaline solution can corrode the crystal surface, form a metamorphic layer containing basic aluminum salts, and cause some changes in the surface morphology. In the lapping interface, the metamorphic layer produced by chemical etching on the surface of AlN crystal is more conducive to the mechanical removal of abrasive particles, which can lay a foundation for improving the lapping efficiency and quality. The result obtained can provide a reference for optimizing the ultra-precision processing technology of AlN crystals.