Ultrathin sulfur-doped g-C3N4 porous nanosheets (SCNNSs) with large lateral size and carbon vacancies were obtained by directly collecting the gaseous product of thiourea under a self-generated NH3 atmosphere. The NH3 atmosphere promoted the formation of sulfur doped g-C3N4 nanosheets with a hierarchical pore structure and a high specific surface area. Carbon vacancies were also generated in the SCNNSs without notably changing the overall chemical structure. The obtained SCNNSs-550 showed a photocatalytic nitrogen fixation rate of 5.99mMh(-1) gCat(-1) under simulated sunlight irradiation within 4 h, which is 2.8 times as high as that of bulk SCN. This superior photocatalytic performance of SCNNSs was attributed to the porous sheet structure with sulfur doping and carbon vacancies, which provide many active sites for surface reactions and increase the charge-carrier separation rate. This novel synthetic method provides a simple and efficient way to dope nonmetals and form a defect structure in g-C3N4 for excellent photocatalytic performance.