The construction of bridge pile foundation in karst area has the disadvantages of large difficulty, high risky, slow period and high cost. For the medium and small span bridges, there is a situation that the costs of bridge pile foundation and karst cave treatment are much higher than that of bridge superstructure. In view of this phenomenon, based on the design example of a landscape bridge in Qingyuan City, the pile foundation in its substructure is eliminated and the enlarged foundation is replaced. And the base stress of the enlarged foundation is reduced through a series of innovative optimization design methods to make the structure safe and reliable. This method avoids the construction difficulties and risks of pile foundation in karst area, shortens the construction period, saves the engineering cost, and provides a new idea for the design of substructure of medium and small span bridges in karst area.