Oriented transfer of electron-hole charge carriers is important during photocatalytic processes. In this study, one-dimensional (1D) tubular B-doped graphitic carbon nitride (g-C3N4) with an effective charge transfer and separation was designed. The doping sites, energy level structure, and photo-generated electron-hole pair separation were predicted using systematical density functional theory (DFT) simulations. The supramolecular precursor for tubular g-C3N4 synthesis, namely melamine·cyanuric acid (M·CA), was controllably synthesized from a single melamine source. Intermolecular hydrogen bonding led to the arrangement of supramolecular aggregate structures into a prismatic crystal architecture during the hydrothermal treatment. The morphology modulation of g-C3N4 from bulk to 1D tubular architecture was realized by calcining the prism-like precursor. B-doped tubular g-C3N4 exhibited a narrower band-gap, multiple reflections of incident light, and oriented transfer of electron-hole charge carriers, which led to a widened light-harvesting range and improved photo-induced electron-hole pair separation and transfer ability. These factors contributed to the photocatalytic activity enhancement towards gaseous NOx degradation under visible light. In this work, a valuable design-fabrication pattern for g-C3N4 modification and engineering via DFT simulations was designed. Moreover, a strategy was developed for the simultaneous foreign atom doping and architecture control of g-C3N4 via the self-assembly of supramolecular precursors.