There are wide range of potential applications about long period fiber grating (LPFG),in order to utilize LPFG as sensing element,it is necessary to develop low cost and high precision wavelength demodulation schemes. An high sensitivity technique for LPFG sensor signal demodulation is introduced. A interferometer based on fiber ring mirror and two LPFGs are designed,the wavelength-shift of sensor grating affected by the measured concentration is converted into variation of time-interval between negative-going pulses that is scanning by F-P tunable filter controlled by PZT. The experiment results show that the sensing sensitivity of this system is 0.04 ms·L /g,with the sucrose solution concentration being from 0 to saturated.