A fiber optic hydrophone based on Sagnac interferometer with saw-tooth active phase biasing technique is proposed and demonstrated experimentally. The influence of 2π reset error of saw-tooth wave on system is analyzed. A phase bias around π/2 is obtained by use of a saw-tooth modulation signal with frequency at 94.05 kHz, and reset voltage of 10.84 V. Acoustic signal is gotten directly by A/D converter trigged by a pulse signal synchronized with saw-tooth modulation signal, and the influce of 2π reset error is depressed. The results show that when the system frequency is 10 kHz, the average amplitude of the signal is 0.067 rad, with variance of 3.08×10-5 rad and ralative variational amplitude of only 0.046%. The phase bias of system is quite steady.