Abstract: A practical and simple immunosensor based on surface plasmon resonance(SPR) technique has been developed to determine the albumin antigen. The SPRdevice was different from popular SPR instrument that modulated angle of incident light at fixed excitation wavelength. The device used in the experimental process is designed on the basis of fixing the angle of incident light and measuring the change of resonant wavelength. The protein A was chosen as the substrate membrane because it can not only form a stable complex with gold, but also form complex with ordinary antibodies. The process of combination between gold and protein Awas monitored, and the kinetic adsoption constant is 23×10⁵L·mol^-1·s^-1. The other combination processed, such as protein Aand albumin antibody, and albumin antibody and antigen had also been studied in real time through observing the change of resonant wavelengths in SPR spectra. The kinetic adsoption constants of all process had been obtained. Using the citrate buffer to rinse the sensing surface, the complex of antibody and antigen could be rinsed out from protein A, and protein Acould be recovered. The relationship between concentrations of albumin antigen and changes of resonant wavelength is linear in the range of 0.02-10 mg/mL. The amounts of albumin antigen in the urine of diabetics have been determined with the SPR sensor. The results are in good agreement with those obtained with standard method.