The invention discloses an 
optical biosensor for detecting 
brain natriuretic peptide (BNP) and a preparation method of a 
reagent thereof, relating to the biosensing technology. A sensor 
reaction zone is a 
capillary channel filled with magnetic nano probe solution, the inner surface of the channel is assembled with a layer of hydrophilic nano material, the upper surface of the channel is an euphotic insulating encapsulation layer with an injection hole, the side surface of the channel is encapsulated by plastics, the two ends thereof encapsulated with plastics are respectively provided with a controllable 
electromagnet. The preparation method of the 
reagent is as follows: preparing magnetic nano probes, fully reacting the magnetic nano probes, a 
brain natriuretic peptide (BNP) sample to bedetected and specificity BNP 
antigen marked by a marker under the united action of the 
electromagnet and the 
capillary channel, forming a compound of magnetic nano probes, BNP 
antigen to be detected and marked 
antibody, and adding substrate for catalytic 
luminescence or exciting luminescent 
quantum dots for 
luminescence. By the sensor detection 
system, the BNP concentration can be rapidly detected. The sensor in the invention can rapidly detect the BNP concentration in a sample, and has the characteristics of high specificity, high sensitivity, a great quantity of saved biological 
reagent andthe like.