The deficient carbon nitride (g-C3N4) displays not only an improved visible light absorption, but also an enhanced photocurrent and restricted electron-hole pair recombination. In this work, nitric acid was used to engineer the defects in g-C3N4, which simultaneously possessed fibrous and layered structures. The modified g-C3N4 (CNx) was utilized as photocatalyst for the effective photocatalytic H-2 evolution. As high as 1160 mu mol/h/g H-2 production rate (lambda > 400 nm) was achieved over CN3, 19 times higher than that of pristine g-C3N4 (CN0, 60 mu mol/h/g). The apparent quantum efficiencies (AQE) of CN3 at 405 and 420 nm were 7.83% and 7.67%, respectively. The mechanism study showed that the surface functionality and the N defects impacted the catalytic efficiency significantly. The O=C, -NHx species and N defects at N-2C site acted as the active sites for the photocatalytic hydrogen evolution, while the -OH group displayed the negative effect in this process. To the best of our knowledge, the active-site structure diagram of the nitrogen-defected CNx for photocatalysis was shown systematically. Additionally, this nitrogen-defected CNx demonstrated a high stability for photocatalytic hydrogen evolution.