A high resolution ultra low frequency phase generated carrier(PGC) demodulation technique for interferometric fiber sensors is proposed. The differential cross multiplication(DCM) algorithm and the Arctangent algorithm of PGC technique are analyzed theoretically and simulated by computer for ultra low frequency demodulation. And the results show that an unnecessary random direct current (DC) drift is induced in the recovered signal of DCM-PGC, but it doesn′t exit in that of Arctangent-PGC. To demonstrate the conclusion, an optical fiber interferometer-based ultra low frequency sensing system is set up, and the comparable experiments of DCM-PGC and Arctangent-PGC are carried out. The experimental results show that only Arctangent-PGC can be used for ultra low frequency demodulation, for it avoids the problem of DC drift as DCM-PGC. At last the ultra low frequency demodulation system is achieved by Arctangent-PGC, and the testing results show that the system has the lowest frequency of 0.01 Hz, a minimum detectable phase shift of 4×10－4 rad/Hz, a dynamic range of 110 dB @ 1 Hz and a linearity of 99.99 %.