Based on analyzing traditional calibration methods, it is found that they are not suitable for the calibration of unencapsulated FBG sensors. Thus a simple calibration method of strain sensitivity coefficient for unencapsulated FBG sensor was proposed. Experiments were carried out to calibrate the unencapsulated FBG sensor with different central wavelength which can cover the demodulation range of demodulator. The type A uncertainty and linearity of FBG sensor were calculated, and the reasons for the difference between calibration result and theoretical value were analyzed. Experimental results show that the proposed method achieves type A uncertainty of less than 3×10-3pm/με and linearity below 0.5% on four FBG sensors.