With the advantages of cost-effective, lightweight, high porosity, high crystallinity and flexible manufacturability, 2D semiconducting polymers have been concerned as appealing candidates for photocatalytic and photovoltaic devices. First principles calculations are reliable in evaluating the fundamental properties of semiconductors including optical gap, band alignments, carrier effective masses and mobilities, and therefore provide insightful perspectives for the design of advanced metal-free polymers for solar energy applications. Here we have summarized the latest advances in the development of promising 2D conjugated polymers as photocatalysts from the theoretical point of view. Constructing 2D conjugated polymers by using appropriate organic monomers to meet the prerequisites of being an effective photocatalyst and conforming to the experimental synthesis possibility is essential in designing and screening potential metal-free photocatalysts for water splitting.