For a hydraulic servo actuator, a fault monitoring method based on model comparison is proposed. This paper expounds the composition and working principle of the actuator, then analyses its failure mode, establishes the mathematical model and simulation model of the hydraulic servo actuator by using the monitoring method based on model comparison, and verifies the simulation model by using the software of matlab, which proves that the simulation model established in this paper is reliable and close to the actual state. It provides a reliable basis for fault monitoring of hydraulic servo actuator.