The invention relates to a device and a method for giving an early warning on freezing of power transformer oil under an extreme cold environment. The method is characterized in that an ultrasonic transmitting transducer is installed outside a transformer tank; an ultrasonic receiving transducer is arranged near the ultrasonic transmitting transducer; the output of the ultrasonic receiving transducer is fed to a prewarning module; an ultrasonic wave with a specific frequency is emitted to the inside of the transformer by using the ultrasonic transmitting transducer; the ultrasonic receiving transducer receives the ultrasonic wave reflected by the internal part of the oil tank, and transmits the waveform and reflection time of the received reflected wave to the prewarning module; the prewarning module judges the freezing thickness of the transformer oil according to the condition set in advance, so as to judge whether the transformer accords with the operation condition. By adopting the method disclosed by the invention, the actual freezing condition inside the transformer can be detected, and a freeze protection strategy for the transformer is provided for substation operation and maintenance workers to support. Thus, correct load dispatching switching is carried out to reduce the effect caused by freezing disasters.