Aiming at the failure phenomenon that the landing gear and the guard plate of an aircraft retracted at the same time,a fault tree was established to analyze and locate the fault,and improvement measures were proposed.The analysis shows that the main cause of the failure is that the large compression stroke of the coordinating valve bolt in the retraction and release process and the gap between the internal spring and the stopper of the product is too large and there is no positioning device.After long time operation of the products,the spring is deflected and the pressed steel ball cannot remains concentric with the bushing,resulting in sealing failure,and finally the guard plate retracts in advance.Aiming at the fault,three improvement measures are put forward.