The invention relates to a method for automatically measuring the diameter of a steel pipe, which is characterized in that the method includes: installing a digital pulse generator, a counter and a calculating unit in the original electrical control device; installing an auxiliary measuring rod capable of moving and positioning along the length direction of idlers on one side of the steel pipe; installing a non-contact magnetic sensor on the auxiliary measuring rod; placing a ferromagnetic sensing head at any position at the circumference of the cutting position; sensing and generating two signals with no contact through rotating the ferromagnetic sensing head once and passing the ferromagnetic sensing head twice; accessing the signals to the counter in the electrical control device; calculating the steel pipe diameter of the cutting position based on the pulse count generated by the digital pulse generator and accumulated by the counter. The method has the advantages that errors caused by the difference between the nominal size and the actual size can be eliminate, and the method has reliable operation, easy maintenance and maintained accuracy.