By using the improved lattice Boltzmann free energy model, the motion simulation of couple bubble with different initial positions and sizes was carried out in high density lithium bromide solution, and the density and velocity vector distribution and motion of bubbles were obtained. Results show that for two bubbles with same size and a constant distance, the rising process consists of two steps, as closing firstly and then separating, however, the initial relative height has no influence on the process. Two bubbles with different size may coalesce or separate, which depends on the position of wake region of large bubble when the small bubble closes to the latter. If the large and small bubbles coalesce, the new bubble velocity decreases.