For fabricating porous carbon nanosheets possessing ultra-high content (ca. 20 at%) of heteroatoms and large specific surface area, we propose a feasible and valid hard template strategy via one-step in situ pyrolysis of crosslinking polymer melamine-formaldehyde-thiourea resins. During the carbonization process, these resins can act as carbon precursor and nitrogen/surfur source with Mg(OH)(2) as template, and the amounts of N/S are facile regulated by adding various quantities of thiourea. As a result, the optimal sample with the molar ratio of thiourea-to-melamine as 2.0 delivers high specific surface area (862 m(2) g(-1)) and large pore volume (1.14 cm(3) g(-1)). More importantly, the N/S contents can reach up to 21.85 at% and 2.33 at%, respectively. Due to the common influence of good porosity and high heteroatom content, it exhibits superior cycling stability (89.4% and 92.2% retention at 3 A g(-1) after 5000 cycle) and remarkable energy density (14.4 and 30.2 Wh kg(-1)) in alkaline and neutral electrolytes, which can intuitively reveal the potential in the application for energy storage device.