The invention relates to a bridge assembly type expansion joint device construction method which comprises the following steps that 1, a temporary supporting piece is built between the ends, close to each other, of two adjacent beam bodies used for building a bridge, wherein the temporary supporting piece comprises a supporting plate and two baffles arranged on the two opposite side walls of the supporting plate correspondingly; the two ends of the supporting plate extend to one ends of the two adjacent beam bodies correspondingly, the baffle is vertically arranged, the lower end of the baffle abuts against the upper surfaces of the beam bodies, and the supporting plate is provided with a positioning piece used for limiting the supporting plate from disengaging from the beam bodies; 2, concrete is poured on the upper surface of the beam body to form a concrete bridge floor, and a reserved groove used for installing an expansion joint device is formed in the beam body area corresponding to the supporting plate; and 3, after the concrete is solidified and cured, when the expansion joint device needs to be installed, positioning of the supporting plate by the positioning piece is canceled, the supporting plate is taken down, the reserved groove is cleaned, and then the expansion joint device is installed in the reserved groove. The method has the effect of improving the negative effect caused by grooving in the prior art.