By high-throughput calculations, 13 thermally and environmentally stable Janus MA₂Z₄ monolayers were screened from 104 types of candidates. The 13 stable monolayers have very high charge carrier concentrations (×10¹⁵ cm^-2), which are better than those of the well-known graphene and TaS₂. Because of their excellent conductivity, the 6 monolayers with band gaps less than 0.5 eV are identified as potential electrode materials for hydrogen evolution reaction applications. For potential applications as photoelectric or photocatalytic materials, bandgaps (E_g-HSE) higher than 0.5 eV remained, which resulted in 7 potential candidates. Based on optical absorption analysis in the visible-light range, H-HfSiGeP₄ and H-MoSiGeP₄ have higher absorption ability and optical conductivity, which is quite impressive for optoelectronic, solar cell device, and photocatalysis applications. Additionally, the transmittance coefficient of Janus MA₂Z₄ monolayers is approximately 70%–80% in the visible-light range, which implies that these monolayers show good light transmittance. For potential applications as photocatalysts, the redox potential and charge effective mass analysis indicate that H-HfSiGeP₄, H-MoSiGeP₄, T-ScSiGeN₄, and T-ZrSiGeN₄ are suitable photocatalysts for CO₂ reduction reactions. Using high-throughput identification, 13 types of new and stable Janus MA₂Z₄ monolayers were explored, and the basic properties and potential applications were investigated, which can reduce the time for experiments and provide basic data for the material genome initiative.