Abstract: The downward motion of the matter within the post-flare loop system of 1984 February 18 is studied under the joint action of solar gravity and the 'both gradients of dipole magnetic field pressure and atmospheric pressure. The two-dimensional line-of-sight velocity field is calculated by means of a numerical method under the assumption that the temperature keeps unchanged within loop system and the loops are quasi-closed. In the system, some parameters as well as their distribution are derived from the curve fit of both velocity fields between theory and observation. The calculations indicate that the effects of density and magnetic field intensity within the loop system on the downward motion of matter is comparatively large while the effect of the temperature is much small.