MoS2 nanosheets were vertically grown in S-doped graphitic carbon nitride (g-C3N4) nanosheets to enhance electrochemical performance. Dicyandiamide was used as a precursor while sodium molybdate and thioacetamide were as additives, and sulfur was introduced into the carbon lattice of thin-layered g-C3N4 by a two-step thermal polymerization. S-g-C3N4/MoS2 (SCN/MoS2) heterojunctions were then created by a solvothermal synthesis. Subsequently, the electrochemical performance of the heterojunctions was explored. The result suggested that the surface of S-g-C3N4 nanosheets load on uniformly arranged layered MoS2 possession revealed the smallest Tafel slope of 77.2 mV dec−1. After 1000 cycles, no significant change was observed for the linear sweep voltammetry curves. The SCN/MoS2 heterojunction revealed a specific capacity of 588 F g−1 after being assembled into an asymmetric supercapacitor. Superior thin g-C3N4 nanosheets and constituting a composite with strong interfacial electronic coupling with MoS2, doping with heteroatomic elements, and changing the surface structure of g-C3N4 enhanced the charge transfer kinetics and improved the electrochemical performance.