Graphitic carbon nitride (g-C3N4) is a rising visible light photocatalyst but limited by its low activity mainly due to the rapid recombination of charge carriers. In the present work, honeycomb-like g-C3N4 was first synthesized via the thermal condensation of urea with the addition of water at 450 °C for 1 h. The morphology of g-C3N4 was changed from a porous honeycomb structure into a velvet-like nanoarchitecture when the condensation time was prolonged. Unlike previous research, the photocatalytic activity of g-C3N4 was decreased with increasing surface area. The honeycomb-like g-C3N4 with a relatively low surface area showed a highly enhanced photocatalytic activity with an NO removal ratio of 48%. The evolution of NO2 intermediate was dramatically inhibited over the honeycomb-like g-C3N4. The short and long lifetimes of the charge carriers for honeycomb-like g-C3N4 were unprecedentedly prolonged to 22.3 and 165.4 ns, respectively. As a result, the honeycomb-like g-C3N4 was highly efficient and stable in activity and can be used repeatedly. The addition of water had the following multiple positive effects on g-C3N4: (1) formation of the honeycomb structure, (2) promotion of charge separation and migration, (3) enlargement of the band gap, (4) increase in production yield, and (5) decrease in energy cost. These advantages made the present preparation method for highly efficient g-C3N4 extremely appealing to large-scale applications. The active species produced from g-C3N4 under illumination were confirmed based on the DMPO-ESR spin-trapping, the reaction intermediate was monitored, and the reaction mechanism of photocatalytic NO oxidation by g-C3N4 was revealed. This work could provide an attractive alternative method for the mass-production of highly active g-C3N4-based photocatalysts for environmental and energetic applications.