The invention relates to a field modification method for a bridge expansion device. An exhaust channel is machined in a side beam of an existing bridge expansion device and penetrates to the upper side face of the side beam from the side face of the side, away from a bridge, of the side beam, and thus when wheels of a vehicle pass expansion joints, especially when the wheels compress air in the joints, the exhaust channel is used for discharging part of gas in the joints, the wheels can hardly compress the air in the joints further, and the air explosion phenomenon generated by the fact that according to existing joints without exhaust channels, wheels compress air and leave immediately, and compressed air releases pressure immediately is avoided; that is, the exhaust channel starts to discharge air when wheels compress air in the joints, immediate pressure reduction of air is avoided, and noise pollution caused by air explosion is avoided.