  In the light of $\phi$-mapping topological current theory, the structure of
cosmic strings are obtained from the Abelian Higgs model, which is an effective
description to the brane world cosmic string system. In this topological
description of the cosmic string, combining the result of decomposition of U(1)
gauge potential, we analytically reach the familiar conclusions that in the
brane world scenario the magnetic flux of the cosmic string is quantized and
the RR charge of it is screened.
