To get the information of underwater acoustic field with the complex acoustic impedance using the fiber optic pressure gradient hydrophone, the calibration must be implemented both for phase of acoustic signal under real impedance and for amplitude of acoustic signal under imaginary impedance. As the impedance of standing wave field is purely imaginary, it can be used for calibration of the amplitude of the acoustic signal. There exists a difference between the acoustic pressure gradient value and the theoretical gradient value because of the unaccordance of sensitivity between the two sensing elements, the directivities like two figures of “8” with one big pattern and one small pattern with an offset of the concave points. The experiment has been carried out by using one dimension pressure gradient hydrophone with two fiber interferometers structure and the results have been showed that the above mentioned result is correct. The ideal directivity pattern can be got by modifying the sensitivity of sensor elements, and the calibration of amplitude can be finally achieved.