Let (M,psi) be a (2n+1)-dimensional oriented closed manifold equipped with a pseudo-free S1-action psi:S1timesMrightarrowM. We first define a textit{local data} mathcalL(M,psi) of the action psi which consists of pairs (C,(p(C);overrightarrowq(C))) where C is an exceptional orbit, p(C) is the order of isotropy subgroup of C, and overrightarrowq(C)in(Ztimesp(C))n is a vector whose entries are the weights of the slice representation of C. In this paper, we give an explicit formula of the Chern number langlec1(E)n,[M/S1]rangle modulo Z in terms of the local data, where E=MtimesS1C is the associated complex line orbibundle over M/S1.Also, we illustrate several applications to various problems arising in equivariant symplectic topology.