Polymeric g‐C3N4 with controllable photoluminescence emission wavelength in the whole visible light range (450–650 nm) is synthesized through the one‐step molecular doping during the thermal condensation process of g‐C3N4 conjugated framework, which opens up its application beyond the conventional catalysis scopes. By adjusting the doped content of hetero‐molecules, the modified g‐C3N4 with the optical properties controlled according to the demand of practical applications can be facilely and largely obtained. It overcomes the limitation of the narrow adjusting range of conventional g‐C3N4 on optical properties and makes it become more promising for applications in solid‐state displays. The corresponding multiple‐color g‐C3N4‐based LED devices and the white‐light LEDs with high quality can be obtained as supported by experiments and theoretical calculations. Moreover, the effect of doped molecule on the π‐conjugated system of g‐C3N4 is systematically studied here, and the tunable luminescence mechanism is proposed. The full‐color photoluminescence emissions (457–650 nm) of g‐C3N4 are first achieved by simple one‐step molecular doping, especially the red‐light emission feature. The corresponding multiple‐color LED devices and the white LED with high color quality are experimentally fabricated with theoretical demonstration of their potential commercial value.