Aluminum-doped propellants are widely used in strategic tactical missiles for their reliability, durability and adaptability. The accurate identification of infrared radiation characteristics of exhaust plumes, as a main means of passive detection, is helpful for rapid warning and tracking. In response to the shortcomings of traditional model that ignores the evolution of particle crystal phases, this paper proposes a radiation calculation model for multiphase Al₂O₃ containing the solid rocket plumes based on the changes of Al₂O₃ crystal structure in high temperature environments. The radiative transfer equation of the gas-solid two-phase plume is solved by using spherical harmonic discrete ordinate method (SHDOM). Compared with the classical method of simplifying the Al₂O₃ particles as single liquid phase particles, the model is more consistent with the results of experimental measurement data, which further improves the calculation accuracy. The infrared spectral radiation characteristics of plumes with different aluminum doping ratios are investigated using the model. The results show that under low aluminum doping ratios, the classical method significantly overestimates the plume radiation in the near-infrared band. At 1.7–2.0 μm, the maximum decrease is 67.2%; in the range of 2.5–3.0 μm, the difference in results between the two methods decreases from 21.6% to 3.6% with the increase of aluminum doping rate; and the particle phase transition in the range of 4.0–4.5 μm does not have much influence on the overall results, whose difference is about 7% on average. Therefore, it is necessary to accurately predict the radiation characteristics by considering the phase change of particles in the plume. These results contribute to the accurate detection and identification of solid rocket motors.