The invention relates to a floating roof tank liquid level measuring device and a measuring method with the device used. The floating roof tank liquid level measuring device comprises a magnetostrictive sensor. The magnetostrictive sensor is composed of a hard sensor measuring bar, a counter weight and a magnetic potential detection electronic bin, the hard sensor measuring bar and a stainless steel guide barrel are fixed together side by side, and the counter weight is provided with a permanent magnet ring core. The floating roof tank liquid level measuring device further comprises a pulley, a pulley support and a steel wire digging rope. The pulley is mounted in a tank top support, the steel wire digging rope passes through the pulley on the support, one end of the steel wire digging rope is fixed to the floating roof of the floating roof tank through a permanent strong magnet on an end, the other end of the steel wire digging rope is provided with the counter weight provided with the permanent magnet ring core and is thrown into the stainless steel guide barrel which is vertically mounted and fixed to the outer wall of a container, and the digging rope is in the tightening state accordingly.