A multiple wavelength demodulation method of long period fiber grating and fiber Bragg grating is proposed. Fiber Bragg grating can reflect the long period fiber grating transmission light. The central wavelength shift of fiberBragg grating will cause the change of reflected light power, and the photoelectric conversion voltage can also change accordingly. So the voltage can demodulate the central wavelength. Because the transmission spectrum of long period fiber grating is very wide, the bandwidth of fiber Bragg grating is so narrow, and a long period fiber grating can demodulate a lot of fiber Bragg gratings. The experimental results show that the voltage of central wavelength and the photoelectric conversion of fiber Bragg grating have a good corresponding relationship. The demodulation method has some advantages including simple structure and high accuracy, so it can be used to measure the parameters of temperature and strain.