The ram air turbine （RAT） actuator is part of the aircraft emergency backup system， and retains little fault detection data； and since it is a complex electromechanical and hydraulic system， it is difficult to determine the cause of the RAT actuator failure. To solve this problem， a fault analysis of RAT actuator barrel based on Modelica simulation is proposed. First， its Modelica model is established according to the structure and working principle of the RAT actuator cylinder， and the accuracy of the model is verified by comparing with the experiment data. The errors in the two working load conditions are 5.3% and 6.3%. Then， the main parameters of the important parts in the model are scanned. A total of four root causes are found for the two types of failures of the RAT actuator that cannot be unlocked and the deployment time is not up to standard. The critical conditions leading to the failure are determined. Finally， a proposal is made to inspect the RAT actuator at the airport. The health status of the actuators can be determined by simple detection of spring elasticity， leakage flow rate， damping hole and matching surface， which realizes the possibility of performance testing of the actuators at the airport， providing a reference for the fault analysis and health monitoring of the RAT system actuator in practice.