For a new class of g(t, x), the existence, uniqueness and stability of 2π-periodic solution of Duffing equation x + cx + g(t, x) = h(t) are presented. Moreover, the unique 2π-periodic solution is (exponentially asymptotically stable) and its rate of exponential decay c/2 is sharp. The new criterion characterizes g x (t, x) −c 2 /4 with L p-norms (p ∈ [1, ∞]), and the classical criterion employs the L ∞-norm. The advantage is that we can deal with the case that g x (t, x) − c 2 /4 is beyond the optimal bounds of the L ∞-norm, because of the difference between the L p-norm and the L ∞-norm.