A combined measurement method on the basis of liquid compressibility and the characteristics of deep sea pressure compensators and pistons is proposed to solve the difficulty in measuring the large pressure of several kilometers of water depth and the small fluctuation pressure of several centimeters of water depth in ocean pressure detection. First, the working principle of the proposed method is introduced. Second, the force of piston is evaluated under static and dynamic pressure measurement conditions, and the corresponding mathematical models are established. Third, the measurement accuracy of static and dynamic pressures is analyzed. Finally, a test platform is built. The results show that the maximum dynamic pressure of 0.025 bar is realized at a pressure changing speed of 0.2 bar/s, and the small fluctuation pressure of 2.8 × 10-3 bars is realized at a pressure of 20 bars. The measurement system can track the changing ambient pressure in real time, and the measurement accuracy of the small fluctuation pressure is only related to the characteristics of the measurement system itself but not to the external large pressure. The accuracy and feasibility of the proposed method are verified. This study provides a new approach for measuring deep sea large pressure and its small fluctuation pressure.