  We formulate a quasiclassical theory ($\omega_c\tau \lesssim 1$ with
$\omega_c$ as the cyclotron frequency and $\tau$ as the relaxation time) to
study the influence of magnetic field on electron-impurity scattering process
in the two-dimensional electron gas. We introduce a general recipe based on an
abstraction of the detailed impurity scattering process to define the
scattering parameter such as the incoming and outgoing momentum and coordinate
jump. In this picture, we can conveniently describe the skew scattering and
coordinate jump, which will eventually modify the Boltzmann equation. We find
an anomalous Hall resistivity different from the conventional Boltzmann-Drude
result and a negative magnetoresistivity parabolic in magnetic field. The
origin of these results has been analyzed. The relevance between our theory and
recent simulation and experimental works is also discussed. Our theory
dominates in dilute impurity system where the correlation effect is negligible.
