The impact of optical source on the accuracy in recovery monochromatic frequency absolute phase demodulation algorithm for optical fiber Fabry-Perot sensor is studied in this paper. The relationship between phase demodulation value changes and the output spectrum of optical source is discussed both in theoretical aspect and demodulation experiment. Results show that the demodulating error has a relationship with the light source optical power. When the temperature changes from low to high, the source spectrum′s redshift causes pressure demodulation value error. The demodulation results show a quadratic relationship with various central wavelength of optical source, which is consisted with the theoretical analysis. Within the pressure range of 140 kPa, the central wavelength fluctuation of light source should be lower than 1.79nm when 0.1% demodulation error is needed.