Abstract：A long period fiber grating(LPG) biosensor for detecting bio-layer thickness is introduced in this paper. The principle of the biosensor is by coating a idiosyncratic bioactive layer(with refractive index n3 ) on the fiber cladding (with refractive index n2 ),and in the condition of n3≥ n2，the variety of bio-layer thickness brings about variety of resonant wavelengths of LPG. So the variety of bio-layer thickness can be detected by measuring resonant wavelengths of LPG. Main function of the biosensor is to detect antigen in blood while antibody as bioactive layer. Reaction between antigen and antibody occurs and it shows that there is antigen in blood when resonant wavelengths shift is detected. Silicify layer is used as fundus layer since its refractive index is much the same as that of antigen and antibody. The thickness of fundus layer is about 120nm. Resonant wavelengths is demodulated by using a turnable F-P interferometer of simple and small frame and practicability.