The utility model belongs to the technical field of power boiler, concretely relates to a device for estimating bellows expantion joint deflection, including base and setting up in the location litterand the locating support of base both sides, be provided with the subassembly that slides between location litter and the base, cup jointed tee bend sleeve and locating component on location litter and the locating support, the rigid coupling has a spring between corresponding and two the telescopic free ends of tee bend of free end between two tee bend sleeves, and the spring middle part is fixed has cup jointed straight sleeve, and straight telescopic length is less than the spring and is in two length between the tee bend sleeve. This a device for estimating bellows expantion joint deflection can simulate and estimate a boiler produced deflection of bellows expantion joint after the thermal expansion to it at the work produced swelling capacity in back that is heated, prevents that bellows expantion joint or pipeline from can take place the contact with the girder steel and causing the inflation to be obstructed to utilize reverse deformation allowance to offset the boiler.