The structure of multi-point fiber Bragg grating (FBG) was adopted to monitor the situation of seepage in dam. It used the wavelength demodulation method which applied InGaAs photoelectric detector array to measure the light intensity. According to the laboratory data, possibility of the multi-point fiber grating sensor system and reliability of the measured data are analyzed. Results showed that the wavelength-temperature response sensitivity of the fiber grating sensor can reach 0.0091 nm/℃ of the temperature field in the dam. Engineering application showed that using the electric heat pulse method to heat the soil around sensors formed the temperature difference with the water, the actual measurement result showed that the method which uses fiber grating to detect temperature difference can apply to monitor seepage. It can realize the positioning of concentrated seepage points in the dam and automatic monitoring. Compared with conventional instruments, the fiber grating sensing monitoring system has obvious advantages in resistance to thunderbolts and disturbances.