Beyond three-dimensional (3D) architectures, polar semiconductor heterostructures are developing in the direction of two-dimensional (2D) scale with mix-dimensional integration for novel properties and multifunctional applications. Herein, we stacked 2D Janus MoSSe and 3D wurtzite GaN polar semiconductors to construct MoSSe/GaN polar heterostructures by polarity configurations. The structural stability was enhanced as binding energy changed from -0.08 eV/-0.17 eV in the N polarity to -0.24 eV/-0.42 eV in the Ga polarity. In particular, the polarity reversal of GaN in contact with Janus MoSSe not only determined the charge transfer direction but also significantly increased the electrostatic potential difference from 0.71 eV/0.78 eV in the N polarity to 3.13 eV/2.24 eV in the Ga polarity. In addition, strain modulation was further utilized to enhance interfacial polarization and tune the electronic energy band profiles of Janus MoSSe/GaN polar heterostructures. By applying in-plane biaxial strains, the AA and AA' polarity configurations induced band alignment transition from type I (tensile) to type II (compressive). As a result, both the polarity reversal and strain modulation provide effective ways for the multifunctional manipulation and facile design of Janus MoSSe/III-nitrides polar heterostructures, which broaden the Janus 2D/3D polar semiconducting devices in advanced electronics, optoelectronics, and energy harvesting applications.