For white light interference method of composite fiber pearl pressure sensor spectral reflectance is complex, its accurate demodulation are susceptible to light source spectrum is unstable and the influence of factors such as noise problem, by theoretical analysis, confirmed the reflection spectral envelope signal from the sensor is sensitive to pressure air chamber, the frequency of cycle is directly proportional to the cavity length,the extraction method of compound fiber pearl pressure sensor reflective spectrum envelope signal, using three parameters of sine fitting method to find the peak position of the adjacent two peak, at the same time, optimization of sinusoidal frequency estimation method reduce the computational burden of the three parameters of sine fitting, through the relationship between the cavity length and signal cycle can eventually get the exact cavity length value.In this paper, a detailed theoretical analysis and numerical simulation of the demodulation algorithm is carried out, and the experiment verifies that the peak value detection method in the spectral method can accurately demodulate the static cavity length of the 80 millimeter-air cavity with a relative error of 0.13% for the composite optical fiber fabry-perot pressure sensor.