The oil-leaking's reasons of a large three phase asynchronous motor in a petrochemical were analyzed.The reason of external leakage is that the pressure in the seal is higher than that of the outside pressure.There is a negative pressure exists near the bearing,which results in that the pressure in the seal is higher than that in the cavity.According to the seal ring wear of motor,an automatic adjustable seal ring was designed.The graphite sealing ring is replaced by the sealing ring made of graphite filled PTEE for the graphite sealing ring is easy to be broken in the seal test.The actual application result shows that the oil spill of the motors bearing seal is resolved by using the automatic adjustable seal ring.