The invention discloses an underground integrally-formed pile column type bridge pier construction method. The method comprises the steps that a stand column segment is subjected to hole drilling at the position of a bridge site in the first place, a stand column steel casing is placed, and a dry hole and the reserved gap in the outer side of the downwards-placed stand column steel casing are filled with mortar; after stand column construction is finished, hole drilling of pile foundation segments is performed, reinforcing cages of pile foundations and a stand column are placed, overall one-time pouring of concrete is performed on the reinforcing cages of the pile foundations and the stand column, and one-time underground forming of the stand column and the pile foundations is completed; after one-time underground forming construction of the pile foundations and the stand column is finished, after pile testing is qualified, construction of other lower portions and upper portions is performed till the train-passing through conditions are achieved, and then construction of digging a road slope is performed till construction is completed. By means of the method, disadvantages of a deep foundation pit and enclosure engineering caused by a traditional method that an existing roadbed is dug in the first place are avoided, safety and reliability of the method that half of an existing road is constructed while half of the road is open to traffic can be guaranteed, design is simple, construction is high in speed, the project cost is lowered, influences on existing traffic are lowered to be the minimum, and the engineering technical level of changing the existing roadbed into a bridge is improved.