The photocatalytic activity of titanium dioxide (TiO2) is largely hindered by its low photoresponse and quantum efficiency. TiO2 modified by conjugated polymers (CPs) is considered a promising approach to enhance the visible light responsiveness of TiO2. In this work, in order to investigate the effect of CP structural changes on the photocatalytic performance of TiO2 under visible light, trimesoyl chloride–melamine polymers (TMPs) with different structural characteristics were created by varying the parameters of the polymerisation process of tricarbonyl chloride (TMC) and melamine (M). The TMPs were subsequently composited with TiO2 to form complex materials (TMP–TiO2) using an in situ hydrothermal technique. The photocatalytic activity of TMP–TiO2 was evaluated by the degradation of rhodamine B (RhB). The results showed that the trend of the structure of the TMP with the reaction conditions was consistent with the visible light responsiveness of TMP–TiO2, and TMP (1:1)–TiO2 had the best photocatalytic activity and could degrade 96.1% of the RhB. In conclusion, our study provided new insights into the influence of the structural changes of TMPs on the photocatalytic activity of TMP–TiO2 under visible light, and it improves our understanding of how conjugated polymers affect the photocatalytic activity of TiO2 under visible light.