This study conducted punching shear tests on steel grid–ultra-high-performance concrete (UHPC) composite bridge decks and performed finite element analysis (FEA) to evaluate their punching shear performance. Initially, a comparative analysis is conducted between the test results and FEA results. The findings indicate that, due to the presence of T-shape steel in this new type of bridge decks, variations in the spacing between adjacent T-shape steel members can lead to two distinct punching shear failure modes: conventional failure mode and unconventional failure mode. Under identical conditions and test specimen parameters, the punching shear capacity associated with the unconventional failure mode is approximately 35% higher than that of the conventional failure mode. Subsequently, a parametric analysis is performed using the FEA method, and the results indicate that, for the composite bridge deck experiencing conventional failure mode, the punching shear capacity decreases approximately linearly with increasing UHPC plate width, whereas it increases approximately linearly with increasing UHPC plate thickness. The type of T-shape steel exhibits negligible influence on the punching shear capacity of the steel grid–UHPC composite bridge deck. Finally, based on the formula for calculating the punching shear capacity of conventional plates in Chinese standards, this paper introduces a correction coefficient that accounts for the width-to-thickness ratio of UHPC plate and proposes an improved calculation method applicable for determining the punching shear capacity of steel grid–UHPC composite bridge decks under conventional punching shear failure mode condition.