In this paper, the morphologies of AlN crystals prepared by the physical vapor transport method under different substrate temperatures and temperature differences were studied. The results show that the growth of AlN crystal is affected by the surface energy of AlN, the average kinetic energy of Al element and the polarity of AlN surface. When the temperature difference is 60 ℃, the growth rate of the (0001) plane of AlN crystal is lower than that of the (10-10) crystal plane, and the AlN belts are observed. Applying the processes to bulk AlN crystal growth, the (0001) surface of AlN crystal exhibites a domain growth mode as the temperature difference at 60 ℃, and the crystal quality is the worst; the (0001) surface of AlN crystal presentes a step flow growth mode as the temperature difference at 35 ℃, and the crystal quality is the best; the (0001) surface of AlN crystal shows a step cluster growth mode as the temperature difference at 20 ℃, and the crystal is easy to crack. AlN substrate with a diameter of 40 mm was finally obtained by process optimization which meets the requirements of device preparation.