Highly crystalline graphitic carbon nitride (g-C3N4) possesses the high separation efficiency of photogenerated electron-hole pairs owing to the significantly decreased intralayer hydrogen bonds, which leads to drastic improvement of photocatalytic activity. However, the preparation of such g-C3N4 material remains a challenge by a simple and economic thermal-treatment in a furnace. Herein, we report a novel and effective strategy for high yield synthesis of extremely active crystalline carbon nitride nanosheets (CCNNSs) by two-step calcination without the assistance of any additive or salt intercalation. As expected, the as-prepared CCNNSs exhibit a remarkably high hydrogen evolution rate of 9577.6 pmol h(-1) g(-1) under simulated solar light irradiation, which is 15.5 times than that of bulk g-C3N4, as well as higher than most of the reported crystalline g-C3N4. Moreover, a highly apparent quantum efficiency of 9.01% at 420 nm for hydrogen evolution can be achieved, which is also superior to the reported crystalline g-C3N4. Such two-step calcination approach not only provides an economical way to effectively regulate the crystallinity of bulk g-C3N4, but also achieves the preparation of CCNNSs with high yield. Our research opens up a new window to self-modification and fabrication of highly active metal-free photocatalysts for solar light-driven hydrogen production.