Ammonium molybdate with lanthanum was prepared by centrifugaldrying, which was calcined to molybdenum trioxide with Lanthanum. The thermal decomposition of ammonium molybdate with lanthanum inairwasanalyzed by XRD and TG-MS, and the morphology and physicochemical properties of molybdenum trioxide with lanthanum were studied at different calcinationstemperature.The result shows that the structure of molybdenum trioxide with lanthanum is amorphous and microspheres withhollowspace inside. There were three stages in the process of the thermal decomposition of ammonium molybdate with lanthanum. Firstly, the centrifugaldrying powder changed from amorphous to crystalline states due to heat-treatment from room temperature to 196.5℃. Secondly, the residual ammonia ion in ammonium molybdate was decomposed from 196.5℃ to 337.8℃, and metastable molybdenum trioxide was formed, finally, metastable β-MoO3 turned into α-MoO3 from 337.8 ℃ to 410.1℃. Molybdenum trioxide with lanthanum prepared at different calcinedtemperature inherited the morphology of centrifugaldrying powder, with the increase of temperature, the surface of powder was coarser and thebreakageofpowder wasseriouser, and the internal small particles of powder changed from irregular and bonded to relatively regular flake. The change of calcinedtemperature had little effect on thecontentsof theimpurityelements in molybdenum trioxide with lanthanum, but with the increasing of calcinedtemperature, the apparent density and particle size reduced slightly.