To study the agglomeration behaviors and agglomeration characteristics of aluminum particles in aluminum-magnesium (Al-Mg) oxygen-poor propellant, the combustion process of Al-Mg oxygen-poor propellants and the microstructure and particle size of agglomeration products of Al particles were investigated by using scanning electron microscopy and optical visualization experiment method. A model for predicting the aluminum agglomeration size was established and fitted to the experimental data. Results show that the secondary agglomeration occurs after the agglomerate of aluminum droplet formed on the combustion surface is separated from the combustion surface. At 1.0 MPa, the propellant specimens burns sufficiently, the aluminum particles after burning become smooth spherical alumina particles and the magnesium particles after burning become white flocculent magnesia. At 0.2 MPa, the propellant specimens burns insufficiently, the aluminum particles are not completely oxidized and the surface is rough. With the increase of combustion chamber pressure, the volume average particle size D(4,3) of aluminum agglomerate decreases, while the surface area average size D(3,2) increases, and particle size distribution tends to single peak, indicating that with the increase of pressure, the closer the values of D(4,3) and D(3,2) are, the more regular the shape of aluminum agglomerate is, and the more concentrated the agglomeration size distribution is. The agglomerate size is inversely proportional to the burning rate.