In the development and utilization of wave energy, the wave energy is first converted to mechanical energy, then to hydraulic energy, and finally to electrical energy. Due to the instability and randomness of the wave energy, which makes the energy easily fluctuate during the conversion process. In this paper, an oscillating wave power generation device based on hydraulic transmission system is taken as the study object, and the factors affecting the stability of the hydraulic transmission system are studied by model experiment and AMESim simulation. The results of the test show that the stability can be improved by optimizing the parameters in the hydraulic transmission system according to different wave conditions; However, the optimized hydraulic system can only ensure that the wave energy within a certain wave condition can be stably converted; when the wave condition changes greatly, the hydraulic transmission system is still easy to cause fluctuations. In addition, a Hydraulic Energy Grading Control System is proposed, which further improves the stability of the wave energy during the conversion process.