Abstract: The experiment on the Giant Magnetoresistance (GMR) effect is a core method for studying the transport properties of spin-polarized carriers, and its measurement accuracy directly depends on the accurate capture of magnetoresistance signals. The geomagnetic field, as an objectively existing weak magnetic field environment, its interference with high-sensitivity GMR effect is often overlooked. This paper focuses on the test path of the coupling direction between the geomagnetic field and the external magnetic field → GMR measurement disturbance, and systematically explores the influence and mechanism of the geomagnetic field in GMR effect experiments. The experiment takes the AA002-02 model of the half-bridge configuration GMR sensor as the research object. By changing the device orientation to control the coupling direction of the geomagnetic field, and comparing the measurement data with and without the action of the geomagnetic field, it is found that the weak geomagnetic field can induce disturbances in the relative resistance change rate of the GMR sensor. This disturbance first strengthens with the increase of the operating voltage of the GMR sensor, and then tends to ease and stabilize. Under the action of an external magnetic field of 1.0788 mT, the maximum disturbance can reach 0.143%, Moreover; this disturbance weakens with the increase of the external magnetic field. Mechanism dissection reveals that the core of this disturbance is that the geomagnetic field slightly changes the relative orientation of the magnetic moments in the ferromagnetic layers inside the GMR device, indirectly perturbing the relative angle between the direction of the magnetic moments in the magnetic layers and the spin direction of the carriers, and ultimately interfering with the magnetoresistance measurement results. Based on this, we propose the core operation steps of "GMR sensor loop orientation calibration" and the experimental optimization strategy of "background magnetic field compensation method". This study clarifies the core mechanism of the geomagnetic field's interference with the GMR effect, and provides theoretical support and practical solutions for optimizing the design of GMR effect experiments and improving the measurement accuracy in the weak magnetic region.