Disclosed is an energy conversion system, comprising a base seat (1), a rotation shaft (2) and an energy conversion device (23). The rotation shaft is fixedly connected to the base seat, and the energy conversion device is provided on the rotation shaft. The energy conversion device comprises a first cylinder (3) and a second cylinder (4), and a first piston (5) fitted to the first cylinder and a second piston (6) fitted to the second cylinder. The first piston and the second piston share a common piston rod (7). The first cylinder and the second cylinder are fixedly connected to each other and are in communication with each other via an air pipe (8). The piston rod is provided with a position limiting device. Furthermore, a buoyancy propelling device (24) is provided on the outside of the piston rod. The piston rod is adapted to the buoyancy propelling device so as to enable the piston rod to move the first piston and the second piston. Further disclosed is an energy conversion method. The energy conversion system and the energy conversion method drive the energy conversion device to rotate by using buoyant energy, and thus convert the buoyant energy into kinetic energy for people to use.