The porous g-CN with nitrogen defects and cyano groups (NC-g-CN) is prepared via an alkali-assisted heat treatment of urea. Alkali can break hydrogen bonds, which accelerate thermal polymerization of urea and formation of nitrogen defects/cyano groups. The presence of nitrogen defects extends the absorption of visible light to longer wavelengths region. The cyano groups can trap g-CN's photoinduced electrons and therefore suppress charge recombination. The formation of porous structure increases the surface area and exposes more active sites. As a consequence, compared to pure g-CN, NC-g-CN shows boosted visible photocatalytic nitrogen fixation activity (1.59 mmol h g) without co-catalysts.