Abstract: Both the experiment method and numerical simulation method are applied in this paper to investigate the energy transformation mechanism of a gas bubble collapseing in the free field. The bubble radius, velocity and acceleration of the bubble evolution process are obtained according to the experimental results via the schlieren method. These parameters are substituted~into the bubble potential energy and kinetic energy equation to explain the energy changing. By using the CFD simulation method, a three-dimensional model with reformulated mass conservation equation and momentum equation considering the weakly compressibility, is introduced to discuss the bubble collapse process. The pressure and velocity distribution around the bubble are extracted from the simulation results in order to analyze the energy transformation mechanism. The results show that (i) the relation between the potential energy and bubble radius maintains the positive correlation, with the increasing of the potential energy, the kinetic energy decreases significantly. The value of potential energy is maximum at the end of the bubble expanding, and the kinetic energy of free field returns to zero at the same time. (ii) During the shrinking stage, a high-pressure area appears around the bubble, and gradients of velocity and pressure are higher than the other area in the free field. The high-pressure area is shrinking gradually when the bubble is collapsing. (iii) The potential energy transforms into kinetic energy during the whole process of the bubble evolution, and the kinetic energy is form of wave energy. A shock wave is captured when the bubble collapse, and the most of bubble energy transform into wave energy of the shock wave at this time.