Abstract: The extended optic fiber hydrophone,which can eliminate the weak correlated flow noise and has directivity and high sensitivity, is suitable for all optic towed arrays. The concept of the extended optic fiber hydrophone is introduced and the model of underwater sound singal detection is developed. The sensitivity and directivity are analyzed theoretically. The design of optic and mechanical configuration of an extended optic fiber hydrophone, which has four same elements, is achieved. The sensitivity and directivity is measured in a basin by the impulse method and the measured sensitivity value is -152dB and the main lobe width of the directivity function measured is 28 degree which axe both agreed with the theoretical values(-151dB and 26 degree).