Procalcitonin (PCT) plays an important role as the factor to reflect patients’ inflammation or sepsis degree. In order to reach a low concentration detection of PCT with label free method, proposed was a type of PCT immunosensor based on excessively tilted fiber grating (Ex-TFG) which is insensitive to temputere but sensitive to refractive index. Staphylococcus aureus Protein A (SPA) was used to modify the surface of Ex-TFG, then highly purified PCT monoclonal antibodies (MAbs) were immobilized on Ex-TFG’s surface through SPA molecules, so as to form the immunosensor which is specific to PCT. Experimental results show that the wavelength shift is about 13pm when PCT concentration is 0.5ng/mL, and it becomes to be 125pm with the concentration of PCT increasing to 200ng/mL. What’s more, the immunosensor becomes saturated at ~1000ng/mL, and the corresponding wavelength shift is about 150pm. Finally, by fitting the experimental data, it demonstrats that the fitting line meets the Langmuir model function with a corelated coefficient of 0.98.