After long-term operation， the occurrence of short-time non-synchronous closing of three phase circuit breaker is inevitable in practice. And because of the existence of delta windings， the three phase transformer will then experience a multi- transient process， which may lead to the high saturation of iron core even with low remanent magnetism， thus result in the mal-operation of differential protection. Based on modeling of a three-phase transformer， the physical process of three-phase non-synchronous closing is analyzed， the changes of flux in the closing of transformer is calculated， and the closing situations which result in the maximum flux is also analyzed， as well as the influence of leakage reactance on flux， which is found to be non-negligible in analysis of inrush. Analysis shows that the second harmonics could be far smaller than the traditional value in case of non-synchronous closing.