The invention relates to a self-adaptive adjusting backing plate device for a prefabricated assembly bridge. The self-adaptive adjusting backing plate device comprises a metal box, wherein the metal box is deformable under action of pressure; the metal box is filled with a polymer steel sand mixture containing a curing agent; the polymer steel sand mixture is in a flow plastic state within a set time and becomes a solid state after reaching the set time; and after the self-adaptive adjusting backing plate is mounted, the polymer steel sand mixture is condensed into the solid state within a certain time and is permanently molded. The self-adaptive adjusting backing plate provided by the invention can automatically adapt to changes of a transverse slope and a longitudinal slope of the prefabricated assembly bridge, and is simple in construction, economical and beautiful.