The active steel approach bridge of floating dock can move over long distances on the pontoon and cause pontoon heeling in the area where the height of water is large.Concerning this issue,pontoon balancing is studied.Using mechanical balance principle,we develope a new pontoon automatic balancing system which can implemente pontoon automatically adjust to keep the balance.We analyse the pontoon balance under three different working conditions of basic mechanical principle,the design method of pontoon automatic balance system and weight calculation principle.Combined with engineering example,we present problems should be noticed in application,e.g.design method and position of balance bearing and track.