Janus transition metal dichalcogenides (TMDs) and their heterostructures offer rich opportunities for property modulation, yet the underlying structural and electronic mechanisms remain elusive. Herein, using scanning tunneling microscopy and spectroscopy (STM/STS), we systematically investigated the structure and electronic state characteristics in lateral and vertically twisted Janus MoSeS/MoSe₂ heterostructures fabricated by hydrogen-plasma-assisted sulfurization. Sulfur substitution induces a 0.2 Å reduction in atomic amplitude and a 0.66 eV widening of the bandgap, which density functional theory (DFT) attributes to the electronegativity of substituted atoms and the intrinsic built-in electric field of MoSeS, respectively. In a moiré superlattice of a 7.4°-twisted Janus MoSeS/MoSe₂ vertical heterostructure, we identify atomic stacking geometries and stacking‑dependent bandgap variations, corroborated by the theoretical simulations. Moreover, the interlayer polarization field arising from stacking further induces nonnegligible modification on the bandgap. This work establishes comprehensive structure-property correlations in Janus MoSeS/MoSe₂ heterostructures, providing a solid foundation for the development of advanced Janus TMDs-based electronic and optoelectronic devices.