  We discuss the influence of the cosmological background density field on the
spherical infall model. The spherical infall model has been used in the
Press-Schechter formalism to evaluate the number abundance of clusters of
galaxies, as well as to determine the density parameter of the universe from
the infalling flow. Therefore, the understanding of collapse dynamics play a
key role for extracting the cosmological information. Here, we consider the
modified version of the spherical infall model. We derive the mean field
equations from the Newtonian fluid equations, in which the influence of
cosmological background inhomogeneity is incorporated into the averaged
quantities as the {\it backreaction}. By calculating the averaged quantities
explicitly, we obtain the simple expressions and find that in case of the
scale-free power spectrum, the density fluctuations with the negative spectral
index make the infalling velocities slow. This suggests that we underestimate
the density parameter $\Omega$ when using the simple spherical infall model. In
cases with the index $n>0$, the effect of background inhomogeneity could be
negligible and the spherical infall model becomes the good approximation for
the infalling flows. We also present a realistic example with the cold dark
matter power spectrum. There, the anisotropic random velocity leads to slowing
down the mean infalling velocities.
