The invention discloses a large generator rotor bearing lubricating oil filtering method which is mainly used for decontaminating lubricating oil of a rotor bearing of a large generator. According tothe method, an upper oil tank 1, a lower oil tank 2 and an oil filtering device 4 are mainly included. The oil filtering method comprises the steps that bearing lubricating oil falls into the lower oil tank from the upper oil tank 1 through an oil discharge pipe 3, then the bearing lubricating oil with pollutants in the lower oil tank is pumped into the upper oil tank 1 through an oil pump 5, anda cycle is formed; the bearing lubricating oil with the pollutants passes through the oil filtering device 4 every time, and the oil filtering device 4 filters out large-particle and small-particle pollutants; when powdery iron powder pollutants pass through a permanent magnetic filter screen 11 every time, a magnetic field of the permanent magnetic filter screen 11 attracts iron powder and enables the iron powder to be attracted to the outer wall of the permanent magnetic filter screen 11 to achieve the effect of filtering out the iron powder; and in this way, through one cycle every time, large particles, small particles, iron powder and other pollutants in the bearing lubricating oil can be filtered out once.