Abstract: Hydraulic test method is an effective way to obtain formation fracture pressure. But the actual hydraulic test curve and the ideal hydraulic test curve are very different, so it is difficult to determine the formation leakage pressure directly from the actual hydraulic test curve. In order to effectively solve this problem, starting from the comparison of the characteristics of ideal hydraulic test curve and the actual one, and based on the similar geometric features between the ideal hydraulic test curve and the underdamping vibration equation with the natural frequency of π/2 and the damping coefficient of 1.00, the paper proposes a method to fit the hydraulic test data by using the underdamping vibration equation to determine the leakage pressure, which could transform the problem of determining the leakage pressure based on the actual test curve into the solving of the initial amplitude of the underdamping vibration equation. Through the analysis on reconstruction of the original hydraulic test data and the reasonable setting of the initial values and the constraint conditions of the fitting equation parameters, the difference of the order of magnitude among test data is effectively eliminated and the stability of the fitting is improved. The case analysis shows that this method provides a scientific basis for formation fracture pressure determination by using the actual hydraulic test curves.