The invention provides an input-current-based method for diagnosing an open-circuit fault of a diode of a subway rectifier set. The method comprises: inputted three-phase alternating currents of fourrectifier bridge grid sides of a rectifier set are collected in real time to obtain three-phase alternating current signals inputted by the four rectifier bridge grid sides; a maximum value of absolute values of all alternating current signals at a current sampling time during the alternating-current power-frequency period is calculated; all sampling current signals during the power-frequency period in which the current sampling time is located are normalized, accumulated summation is carried out, calculation is carried out to obtain a normalized sum value of 12 inputted alternating current signals; and if the normalized sum value of alternating current signals during the continuous five periods exceed a positive threshold Sup or a negative threshold Sdown, occurrence of an open-circuit fault at a diode of a lower bridge arm or upper bridge arm of a corresponding phase of the corresponding rectifier bridge is determined. Therefore, the open-circuit fault of the diode can be detected and the positioning of the fault diode is realized; maintenance is carried out timely and conveniently; and losses increasing and accident occurrence are avoided.