Abstract: Stable, strong magnetic fields are instrumental in scientific research, biomedicine, and material science. Addressing the challenges in measuring and tracing super-strong magnetic fields, we introduce a measurement method derived from the magnetic flux modulation principle, leading to the development of a highly stable magnetic field measuring instrument. By employing a refined signal conditioning circuit and harnessing the phase-locking principle to achieve optimal signal-to-noise ratio, we deduce the induced electromotive force corresponding to the magnetic induction intensity of the inspected strong magnetic field. Calibration of the instrument's coil constant with a standard magnetic field allows the constant to serve as a transfer standard, facilitating ultra-strong magnetic field measurements and calibrations. We examined superconducting strong magnetic fields within the 1-7 T range, providing an in-depth analysis and evaluation of the measuring device and coil constant. Our findings highlight that the measurement bias for the 1-7 T magnetic field is better than 0.083%. The relative uncertainty introduced by our instrument is up to 3×10⁻⁴, marking a significant advancement for ultra-strong magnetic field measurements.