Noncanonical G-quadruplex nucleic acid structures can be used as probes in biosensors for the detection of metal ions, proteins and nucleic acids. Angiotensinogen (AGT) is a glycosylated globulin found in serum, which can regulate blood pressure and body fluid homeostasis. AGT is an important part of the renin-angiotensin system (RAS) and can be potentially used as a biomarker of diseases with RAS alterations. G-quadruplex based biosensors can detect targets with high accuracy and speed and low cost. Employing the magnetic bead enrichment method we constructed a reliable and efficient fluorescent biosensor platform for G-quadruplex based detection of the human AGT protein. The primary antibody in our biosensor is recognized by fluorescently labeled secondary antibodies, which leads to the detection of AGT captured by the G-quadruplex aptamer coupled magnetic beads. This G-quadruplex based fluorescent biosensor designed with a detection limit of 5 × 10-5 mg mL-1 was used for the successful detection of AGT at the cellular level. Our G-quadruplex based fluorescent biosensor will contribute to the more reliable and efficient detection of AGT.