Tunable filtering method based on F-P cavity is a distribution fiber grating sensing demodulation method with good practicability and mature development. Peak detecting technology is the key to this method. Spectrum correlation method is emphasized to realize the principle of peak detecting. Waveform interference among sensing signals of distribution fiber grating sensing system is analyzed. Based on TMS320C6416 fixed point digital signal processing (DSP) chip, a correlation algorithm of relative frequency is realized and the algorithm has an ideal result in both calculation precision and operation speed. Research on the technology has certain theoretical guidance meaning and reference value.