In this paper, we consider the existence of periodic solutions for a class of semi-linear second-order differential equations. The difficulty of traditional methods lies in a mass of detailed estimates. By applying the viscosity solutions method and the classical upper-lower solutions method, as well as the Leray-Schauder fixed point principle, we establish the existence of periodic solutions to the equation under some weaker conditions. Our result improves and generalizes many results on the ropes mechanics equations in the previous literature.