The accuracy in the measurement of magnetization curve ( $M$ – $H$ ) for soft magnetic materials is easily influenced by the external static field in the unshielded environment. To eliminate the influence from the environmental field, the measurement system must be placed in a zero-gauss chamber. However, such a device is not widely available and also costly. In this work, the rapid $M$ – $H$ measurement system with a giant magnetoresistance (GMR) sensor serving as the zero-field detector is proposed. The $M$ – $H$ curve is obtained by integrating the voltage in the pickup coil to determine the magnetization of the test sample under a sweeping magnetic field. The effect of the environmental static field is eliminated by using the output of the GMR sensor as the trigger signal to determine the offset field in the measured $M$ – $H$ curve. The experimental results show that the $M$ – $H$ curves for various kinds of soft magnetic samples, e.g., mu-metal wire, can be obtained under the interference of an applied static field. The proposed method helps improve the accuracy of the rapid $M$ – $H$ measurement system in the unshielded environment.