The invention discloses a method for monitoring the operating state of a power transformer on line under the direct-current magnetic bias condition. The method comprises the following steps that a vibration signal of the wall of a transformer tank, a three-phase voltage signal of the transformer, a three-phase current signal of the transformer and a neutral-point direct current are collected; based on the K-means cluster analysis method, a transformer vibration cluster model is built; the amplitude of the 100Hz component of the measured vibration signal of the wall of the transformer tank is compared with the output quantity of the transformer vibration cluster model, and if the relative error between the amplitude and the output quantity is smaller than a threshold value, it is determined that the operating state of the transformer is normal; if the relative error between the amplitude and the output quantity is greater than or equal to the threshold value, it is determined that the operating state of the transformer is abnormal. The method can be used for effectively and highly sensitively monitoring the operating state of the transformer on line under the direct-current magnetic bias condition, and therefore transformer direct-current magnetic bias effective measures can be taken timely, alternatively the transformer can be maintained timely, and major faults are avoided.