The invention discloses a power battery fault diagnosis method and system. The method comprises the following steps: establishing a voltage matrix of a battery cell, extracting a sub-matrix interval of the voltage matrix, establishing a median matrix according to the median of the battery cell voltage in the sub-matrix interval, establishing a voltage deviation matrix according to the sub-matrix interval and the median matrix, calculating a voltage deviation increment according to a voltage deviation value in the voltage deviation matrix, establishing a voltage deviation increment matrix, accumulating the number of times that the voltage deviation value in the voltage deviation matrix exceeds a set interval, establishing a voltage deviation accumulation number matrix, establishing a jointmatrix, performing two-dimensional clustering on the established joint matrix by adopting a DBSCAN density clustering method, and judging whether the battery cell has a fault or not according to a clustering result. According to the power battery fault diagnosis method and system provided by the invention, the fault judgment efficiency of the battery cell can be improved, and the judgment accuracycan be improved.