Approximate symmetries and infinite series symmetry reduction solutions to perturbed Kuramoto-Sivashinsky equation. Starting from Lie symmetry theory and combining with an approximate symmetry method, and using the package LieSYMGRP proposed by us, we restudy the perturbed Kuramoto-Sivashinsky (KS) equation. The approximate symmetry reduction and the infinite series symmetry reduction solutions of the perturbed KS equation are constructed. Specially, if selecting the tanh-type travelling wave solution as initial approximation, we obtain not only the general formula of the physical approximate similarity solutions, but also several new explicit solutions of the given equation.