The interaction between the anti-Ractopamine (Rac) monoclonal antibody and the Rac derivation immobilized on the sensor chip surface was studied with surface plasmon resonance (SPR) biosensor. A continuous detection method based on the linear response during the association phase was developed. The sensor chip surface was regenerated after several tests performed continuously, thus the detection step was simplified and the life span of the chip was prospected to be prolonged. The detection was performed as an inhibitive immunoassay. The mixture of anti-Rac monoclonal antibody and the sample flowed over the surface with Rac derivation was immobilized. The relative response was in inversely proportion to the concentration of Rac. The detection limit was less than 4 μg/L with a detection time of 15 min.