As an index of maintaining low fuel consumption, the digging force of an excavator is affected fully by the performance of the hydraulic actuators including boom cylinder, stick cylinder, and bucket cylinder. The seals are used widely in a fluid power system to prevent fluid leakage. However, wear induces changes in the diameter of the seal's cross-section area, having an effect on the sealing capability of seals. This article proposes a model for performance degradation induced by wear of a hydraulic actuator of an excavator. The model in this article describes the physical process of performance degradation of a hydraulic actuator by analyzing piston response when wear occurs. The model includes a dynamic model of a hydraulic actuator, a model of squeezing stress and deformation of a compressed elastomeric O-ring seal, a wear model of a seal, and a leak rate model. These models can be used for deducing the laws of performance degradation of a hydraulic actuator of an excavator and for predicting the useful life of a key component such as the seal. Finally, based on the established model, simulation results of a hydraulic actuator’s response are provided.