In this paper, the branch ratio of η→ππγ and η→3π is studied through the analytic continuation of a dispersive relation with respect to the mass of η. It is pointed out that the anomalous threshold and complex singularities are both present. Under appropriateapproximation, we obtain the branch ratio R ((η→ππγ)/(η→3π))≈0.65, which is close to recentexperimental results.