  The nullity of a graph is the multiplicity of the eigenvalues zero in its
spectrum. A signed graph is a graph with a sign attached to each of its edges.
In this paper, we obtain the coefficient theorem of the characteristic
polynomial of a signed graph, give two formulae on the nullity of signed graphs
with cut-points. As applications of the above results, we investigate the
nullity of the bicyclic signed graph $\Gamma(\infty(p,q,l))$, obtain the
nullity set of unbalanced bicyclic signed graphs, and thus determine the
nullity set of bicyclic signed graphs.
