This paper studies the diffusing characteristics and the concentration distribution of unsymmetrical dimethylhydrazine (UDMH) released on the ground and formed into a pool under the storage condition due to some causes. The change process of concentration of UDMH evaporated from the pool on the ground is numerically simulated with Fluent software by the Computational Fluid Dynamic (CFD) method. The movement of the storeroom-air and the space-time distribution characteristics of the concentration of UDMH evaporated continually under some specified aeration conditions are studied. It is shown that the effect of gravity on the dispersion is significant; under the condition of air exhaust by devices and air intake by subatmospheric pressure, the storeroom-air moves in cycles; high concentration areas are mainly under the air intake where the speed of air is low; in the cutter tip of z axis, UDMH is distributed in circles. These results can provide some food of thought for accident disposal.