  In light of $\phi$-mapping topological current theory, the inner topological
structure of Hopf invariant is investigated. It is revealed that Hopf invariant
is just the winding number of Gauss mapping. According to the inner structure
of topological current, a precise expression for Hopf invariant is also
presented. It is the total sum of all the self-linking and all the linking
numbers of the knot family.
