Bubble rising process is a complex and important phenomenon on the nuclear energy safety. Based on the interface diffusion and the finite element method, the shape and velocity of the large bubble rising behavior are simulated. The results agree well with the experiments of the references. It is indicated that the diffuse interface method can correctly simulate the motion characteristics of the bubble. By using this model, the shape, velocity, and oscillation of bubbles with different initial diameters are analyzed during the rising process. Moreover, when the bubble diameter is 15 mm, the variations of shape and velocity during the rising process in different sizes of the channel are also studied. The results show that with the increase of the initial diameter, it takes more time to get the stable shape of the bubble from the ellipsoid to the cap type. The larger initial diameter bubble is, the greater velocity at the top of the bubble. The velocity at the bottom of the bubble increases rapidly at very beginning so that the bottom of the bubble inward depression. Then, the bottom velocity falls back and shocks around the top velocity of the bubble. When the bubble rises in a smaller size channel, the top velocity of the bubble is reduced. The height/width ratio is increased, and it needs more time to get the stable shape.