In order to avoid the complicated control and fussy procedure associated with foreign species and templates in conventional synthesis strategies, a simple reactant conversion-intercalation strategy is developed to synthesize interlayer-expanded MoS2 (E-MoS2) by employing ammonium thiocyanate converted from a thiourea reactant as intercalator. In this strategy, the thiourea plays a bifunctionality role as reactant and intercalator precursor to ensure in situ embedding into the interlayers of MoS2 to expand the interlayer spacing. The optimal E-MoS2 obtained presents superior supercapacitor performance with a specific capacity of 246.8 F g-1 at 0.5 A g-1 in 1 M Na2SO4 electrolyte in a three-electrode system, outperforming pristine MoS2 prepared by a conventional hydrothermal method (42.5 F g-1 at 0.5 A g-1). Furthermore, a symmetric supercapacitor based on an E-MoS2 electrode delivers a high specific capacity of 261.3 F g-1 and energy density of 13.3 W h kg-1 at 0.5 A g-1, and excellent cycling life with 81.7% capacity retention after 3000 cycles at 2 A g-1. Density functional theory calculations reveal that the NH4+ and SCN- can be effectively adsorbed on the surface to be inserted into the interlayers during the growth of MoS2, resulting in an expanded interlayer spacing of 9.4 Å, and the favorable electrochemical performance stems from the large Na+ adsorption capacitance and low diffusion barrier of the E-MoS2. This work offers a novel intercalation strategy that may be generally applicable to other layer-structured materials, shedding some light on the development of high-performance electrode materials via interface engineering for energy applications.