In this paper, unsteady analysis was conducted on the space distribution and depositing process of the solid particulates produced by aerosol fire suppressants under fire conditions. How the particles diameters affects their movement was demonstrated.The features of the particle diameters affecting the movement status were presented. The stability and resort time for the solid particles in the flow filed was explored. And the expression of the criterion (critical) diameter for a particle depositing to the fire source surface was obtained. The results demonstrated that: the depositing process of the solid particulates and the depositing Renault number are related to the particle diameter, and the flow is almost in the relatively steady flow status with low Renault number. The depositing velocity is proportional to the square of the particle diameter. When the particle diameter is lower than 100 μm, the flow will reach the steady flow status in very short time, and while the particle diameter reaches 1 μm, the particles can suspend in the space for a long time, and can be entrained into the flame region showing the extinguishing effect.