The principal of fiber optic gradient hydrophone is the finite difference of pressure gradient; whilst the technology of fiber optic acoustic pressure sensor is relatively complicate which will cause the imbalance of the two difference terms that may finial lead to measurement error. The impact of the sensitivity difference on the measurement error of gradient hydrophone is studied. It shows that the measurement error lead by sensitivity difference is more serious in actual circumstance of plane wave field, thus the traditional standing wave tube calibration of the gradient hydrophone will generate phase offset in pressure and pressure gradient measurement. To put up with the problem, a new fabrication scheme of fiber optic gradient hydrophone based on two interferometers in one dimension is proposed and analyzed; some additional measurements are needed to cooperate with the new scheme. Using this calibration flow, the sensitivity difference is conquered and fiber optic pressure gradient hydrophone can be utilized in actual application.