Carbon nitride is a promising metal-free visible light driven photocatalyst and sustainable material for address contaminant pollution and water splitting. However, the insufficient visible light absorption and fast charge recombination of carbon nitride have limited its practical application. Herein, the self-assembly carbon nitride (denoted as TCN) by molecular doping copolymerization of urea and 2-thiobarbitucid acid (TA) was prepared. XPS and elemental analytical results indicated that TA was doped in the framework of carbon nitride successfully. The self-assembly copolymerization would result in the change of morphology, intrinsic electron and band structure of carbon nitride. Theoretical calculations and experiments confirm that the band gap of TCN could be adjusted by changing the amount of 2-thiobarbitucid acid. Moreover, the efficiency of charge carrier transfer and separation was greatly enhanced. As a result, the optimized photocatalyst TCN-0.03 exhibited superior activity with a high reaction rate of 0.058 min(-1) for the degradation of sulfamethazine under visible light irradiation, which is 4.2 times higher than that of urea based carbon nitride (U-CN). As a multifunctional photocatalyst, TCN-0.03 showed enhanced activity for hydrogen production (55 mu mol h(-1)), which was 11 times higher than UCN. The apparent quantum efficiency reached to 4.8% at 420 nm. A possible mechanism was proposed to explain the photocatalytic reaction process. This work provides insight into the rational design of modified carbon nitride by other organic monomers copolymerization to enhance the photocatalytic activity.