The invention discloses a preparing method of sheet aluminium nitride and belongs to the technical field of ceramic powder material preparation. The method comprises following main steps of: preparing an aluminum source, an amine type organic compound, a water-soluble carbon source and assistant agents which are adopted as raw materials into a solution according to a certain ratio, heating the solution, volatilizing the solution, concentrating, decomposing to obtain precursor powder, reacting the precursor powder at 1600-1900 DEG C in a certain atmosphere for 2-10 h, and heating the reacted powder in air at 600-800 DEG C for 2-4 h to obtain the sheet aluminium nitride having a particle size of 3-10 mum and a radius-thickness ratio of 25-90. The method is simple in process, low in cost and easy in industrial production.