  Spiral wave, whose rotation center can be regarded as a point defect, widely
exists in various two dimensional excitable systems. In this paper, by making
use of \emph{Duan's topological current theory}, we obtain the charge density
of spiral waves and the topological inner structure of its topological charge.
The evolution of spiral wave is also studied from the topological properties of
a two-dimensional vector field. The spiral waves are found generating or
annihilating at the limit points and encountering, splitting, or merging at the
bifurcation points of the two-dimensional vector field. Some applications of
our theory are also discussed.
