A biosensor based on grating waveguide coupling resonance angle has been designed. It achieves accurate detection for adsorbate on sensor surface through the change of grating waveguide mode spectrum to detect the change of optical refractive index on the sensor surface. The theoretical models of the threelayer structure and fourlayer structure have been derived on the basis of the plate medium optical waveguide theory. The relationships among the incident angle and the refractive index of the detected solution and the molecular membrane layer thickness have been drawn through the experiment. The results show that there is a good linear relationship between the incident angle and the refractive index of the solution to be measured, and the sensor has high sensitivity of 0.01 (°)/nm. The proposed method can be used to fabricate the labelfree biosensor for the biological molecule diction, particularly for protein molecular biological diction.