The invention discloses an expansion device for bridges, solving the problems of short service life, complex maintenance operation and high cost in the prior art. The expansion device for bridges adopts the following technical scheme that the structure comprises an elastic expansion mechanism and a matched assistant positioning device, wherein the elastic expansion mechanism is arranged on the gap between the bodies of two adjacent bridges and is composed of organ plates made of an austenitic stainless steel cold-rolled sheet material. The expansion device for bridges has the beneficial effect that the elastic expansion mechanism is made of austenitic stainless steel, has the advantages of good acid and alkali resistance, good drainage performance and good construction performance and greatly strengthens the fatigue resistance by means of the structure of the organ plate.