The invention provides a method and system for diagnosing the fault type of a photovoltaic array. The method comprises the steps of judging whether the photovoltaic array has a fault or not based on power data of photovoltaic assemblies in the photovoltaic array and preset power data of the photovoltaic assemblies when the photovoltaic array is normal; if there is a fault, calculating photovoltaicarray distinguishing factors based on the operating state of bypass diodes respectively connected with the photovoltaic assemblies in parallel; and determining the fault type of the photovoltaic array based on the photovoltaic array distinguishing factors. The method determines the photovoltaic array distinguishing factors and diagnoses the fault type of the photovoltaic array by using the powerdata of the photovoltaic assemblies and the operating state of the bypass diodes, can diagnose the fault location in real time, effectively improves the accuracy of the fault type of the photovoltaicarray and ensures the safe and reliable operation of the photovoltaic array.