Aiming at the phenomenon that the wet joints of precast bridge decking are damaged by impact load during service, a functional gradient composite structure design concept was introduced to design a wet joint of functional gradient of precast bridge decks. Taking the relative functional layer thickness and the compressive strength of the UHTC layer as the main variation parameters, the impact resistance of wet joint model specimens was studied by using drop-weight impact test. The results show when the compressive strength is constant, the impact life increases exponentially with the increase of the relative thickness of the functional layer, and the prediction model of the impact toughness of wet joint model specimens about the relative functional layer thickness is established. For the model specimens with the same thickness of UHTC, the impact energy consumption decreases with the increase of the compressive strength of UHTC layer, and the reduction range increases with the increase of the compressive strength of UHTC layer. There is a linear correlation between the initial crack life and the final crack life with different compressive strengths. According to the grey correlation analysis, the degree of influence of the three factors on the fracture life is ranked from strong to weak as follows: initial crack life＞relative functional layer thickness＞compressive strength＞0.5. On the basis of the above, two prediction models of failure impact life, including Model_Ⅰ and Model_Ⅱ，are established. The results show that the prediction accuracy of Model_Ⅰ is high and its applicability is good, while the accuracy and applicability of Model_Ⅱ are poor.