Abstract: The principle of SPR(Surface Phasmon Resonance), the energisation condition of SPR phenomenon and the phase variation of the reflected light in SPR are briefly presented.The application of transverse Zeeman laser is also analyzed.The equipment based on transverse Zeeman laser is set up to detect the phase variation in biomolecular interaction and a lot of experiments are made upon the interaction of ricin and its antibody.The experiments show that the optical path of P light coincides with that of S light,which is self-adaptive to the disturbance of the air index or any others and has high stability. The phase detection has higher reliability than the intensity detection, with the precision of laser frequency stability up to 3.5×10-10.Transverse Zeeman laser is the ideal light source of phase detection in biomolecular interaction based on SPR principle.