The brake-by-wire system is widely applied into small and medium unmanned aerial vehicle(UAV). The composition and principle of the brake-by-wire system were introduced. The failure phenomenon that the electrohydrostatic system was out of control without instruction when it was used on board was introduced. A new failure analysis method was presented through the deep study of the fault. The fault was located by methods of visual inspection, PCB X-ray examination, industrial CT inspection, simulation calculation and analysis, test verification, and the mechanism was analyzed. The fault simulation test and the recurrence of the fault phenomenon were carried out, and the fault causes were verified. According to the cause of the failure, the solution measures were put forward to prevent the failure.