A biosensor for detecting an influenza A virus in a sample is disclosed, which includes: an influenza A virus antibody immobilized on a surface of Au—Fe3O4 composite; where the antibody binds with the influenza A virus in the sample, which converts 4-methylumbelliferyl-N-acetyl-α-D-neuraminic acid (MUNANA) to 4-methylumbelliferone (4-MU), where the 4-MU emits green light at pH of 5.5-6.5; and wherein the 4-MU emits blue light at pH of 9.3-11.3. In the biosensor, 1,1′-oxalyldiimidazole chemiluminescence (ODI-CL) reagent may be utilized to emit the blue and green lights.