In this paper, a fine pure Magnesium aluminate spinel powder was synesised by Gel solid reaction,using Al(NO3)36H2O and Mg(NO3)26H2O as raw materials under given condition. The precursor was formed by polymer gelling.The phase changed according to heating temperatures from 700°C to 900°C.The single phase of Magnesium aluminate spinel was obtained by calcining the precursor at temperatures higher than 900°C. The phase changing process and relationship among calcination parameters, phase composition and image of particle are studied to determine the optimal conditions for the synthesis of fine Magnesium aluminate spinel powder. The powder obtained was characterized by XRD,TEM etc. The results of analysis showed that the ultrafine powder is of spinel phase, high purity, good dispersion.This process for preparing fine Magnesium aluminate spinel powder was very simple and inexpensive.The results of analysis showed that the ultrafine powder was of mullite phase, high purity, good dispersion, and the particle were homogeneous.