Magnetoresistive sensors, which are based on the magnetoresistance effect, use magnetic nanoparticles (MNPs) as biological signal carriers. They provide a technical pathway for biodetection with high sensitivity, strong anti-interference capability, and ease of miniaturization. This article first systematically explains the physical mechanisms and core device characteristics of anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), tunneling magnetoresistance (TMR), and giant magneto-impedance (GMI) effects, with a focus on analyzing the signal conversion and amplification processes in biosensing using MNPs as labels. Combined with detection examples of typical targets such as proteins, nucleic acids, cells, and microorganisms, the performance characteristics of different magnetoresistive sensing platforms in terms of sensitivity and application scenarios are compared and analyzed. The influence of physical factors such as spin polarization, interface scattering, and low-frequency noise on the detection limit is also deeply discussed. The analysis shows that GMR and TMR technologies achieve the best balance between sensitivity and integration, while TMR and GMI devices demonstrate great potential for achieving ultra-high sensitivity detection. Finally, this article summarizes the key challenges currently faced by magnetoresistive sensors in terms of long-term stability, system-on-chip integration, and direct detection in complex biological samples, and looks forward to the development directions of magnetosensing technology in areas such as high-throughput screening, point-of-care testing (POCT), and wearable health monitoring.