The deck of the second side span and middle span of a cable-stayed bridge adopts a new combination system of "8 mm flat steel plate + 15 cm UHPC layer + PBL shear connection". The UHPC of the prefabricated section of the bridge deck is connected by stepped dovetail wet joints. In order to study the static negative bending moment performance of the wet joint of the composite bridge deck，full-scale model specimen was made for the test. Through the application of load，the displacement and strain of bridge deck specimens under all levels of load were measured and the cracks were observed. The the load-displacement curve，load-strain curve，crack development law and failure pattern were obtained. The results show that the crack resistance and bearing capacity of the stepped dovetail wet joints are about the same as those of the non-joints of the wet joints. The stiffness gradually deteriorates after cracking，When the crack width is 0.2 mm，the relation between crack and load changes from linear to nonlinear. The bridge deck still maintains good ductility after reaching the ultimate load. PBL shear connection can ensure the integrity and common deformation of steel and UHPC. The numerous and dense cracks of bridge deck are mainly concentrated in the cast-in-place section. If the strain of steel bar and concrete in the middle section of the span are larger than that in other parts，the failure of the specimen will be marked by the sharp increase of crack width in the middle of the span.