The invention proposes a method for preventing differential protection from failing due to faulty inrush current on the converter transformer valve side. The differential current of a fault phase is greater than the starting current 0.2IN (IN is the rated current), and the second harmonic percentage content thereof is calculated after one cycle. If the second harmonic content is less than 15%, theprotection operates. If the second harmonic content is greater than 15%, and the current of the valve side of the converter transformer fault phase has direct component polarity reversal after the fault, the protection operates. If the harmonic content is greater than 15%, but the current of the valve side of the converter transformer fault phase does not have direct component polarity reversal after the fault, the protection does not operate. According to the invention, the feature of polarity inversion of the direct component of the current of the valve side of the fault phase after the occurrence of a switching fault is used; based on the principle of second harmonic braking, new criteria for preventing differential protection rejection caused by faulty inrush current caused by the ground fault of the converter transformer valve side is prevented; the problem of blocking of converter transformer differential protection caused by the fault in an area outside the converter valve sidegenerating area is effectively solved; and the reliability of converter transformer differential protection is ensured.