Abstract: The pneumatic-hydraulic actuator of shut-off ball valve on a gas trunk line is often shut off by mistake, severelyinfluencing pipeline safety operation and enterprise economic benefits. This paper systematically analyzes the abnormalshut-off accidents using fault tree quantitative analysis method. Above all, the principle of automatic shutoff of pneumatic-hydraulicactuator is illustrated, then the possible factors causing abnormal shutoff of the actuator are analyzed thoroughlyusing fault tree method, and finally the relevant influential degree of the factors are calculated quantitatively. It is knownbased on the quantitative analysis of 29 basic incidents probably inducing abnormal shutoff that the main factors causingthe shutoff of the pneumatic-hydraulic actuator by mistake are artificial error, ESD false triggering, power disconnection ofelectromagnetic valve and ice blockage of pressure pipeline, etc.. Fault tree analysis results are consistent to real pipelineoperation situations, indicating rationality and effectiveness of fault tree analysis method in real production and application.