The aerial working platform expansion bridge and steering system working principle was analyzed, the expansion bridge and steering system hydraulic system fault tree was established on the basis of it, the causes of failure was analyzed, and hydraulic circuit key parameters was monitored by using sensors, EPEC2023 controls and displays EPEC2025. The analysis results show that the method can improve the diagnostic efficiency, shorten the time of diagnosis, rapid rule out the failure in the process of the aerial working platform at work.