Abstract: Direct numerical simulation of FTM (Front Tracking Method) was used to study free rising motion of single bubble in a tube with fins in gravity. Ratio of fin height and pitch to tube length of a side are chosen as geometric characteristics. Morton number is used as characteristic parameter of fluid. With analysis on free rising motion of bubbles at different fin height, pitch and Morton number effects of geometric characteristics of tube and parameters of fluid on motion trajectory of bubbles are studied. It shows that under effect of fins bubble shape is asymmetry and bubbles have lateral displacement like a "snake".This phenomenon is associated with geometric structure of fin and fluid around bubble. The higher fin height is, the more obvious phenomenon becomes. The smaller the viscosity of fluid around bubble is, the greater the horizontal displacement is.