Fault tree was established for abnormal opening failure of a certain type of 2D directional valve during using without power supply. Using the failure analysis method, the fault location and fault cause analysis were carried out, and the fault recurrence test was carried out. The cause of the fault is that the gas precipitated from the oil enters the sensitive cavity of the 2D directional valve, which reduces the volume elastic modulus of the oil in the sensitive cavity; when supplying oil to the 2D directional valve, the pressure of the high-pressure chamber of the 2D directional valve is established faster than the pressure of the sensitive chamber, causing the spool to move beyond the critical position of failure, so the sensitive chamber is connected to the oil return chamber, and the spool is kept at the limit position. Based on the analysis results, limit measure was added to the end cap of the 2D directional valve. The test proves that the measure is effective.