Due to the broken of inversion and mirror symmetry, Janus structures are believed to have abundant properties and can be applied in two-dimensional (2D) materials, but the research of 2D Janus materials is insufficient. In this work, Janus materials ZnXY2 (X = Ge, Sn, Si and Y = S, Se, Te) monolayers are designed referring to group-III monochalcogenides. The electronic, mechanical and piezoelectric properties and stabilities of ZnXY2 monolayers are investigated by the first-principles calculations. The electronic property shows that ZnXY2 monolayers are semiconductors with wide direct band-gaps and large effective masses difference between holes and electrons. The mechanical property shows that ZnXY2 monolayers possess low Young’s modulus, bending modulus and ductile properties, which is beneficial for applications in flexible nanodevices. The piezoelectric property shows that, compared with conventional Janus group-III monochalcogenides and bulk piezoelectric materials, ZnXY2 monolayers exhibit higher in-plane and comparable out-of-plane piezoelectricity. The further analyzations indicate that the distances and electronegativity differences between atoms are two important influence factors to piezoelectricity. In summary, the flexibility, piezoelectricity, direct band-gaps and large effective mass difference make ZnXY2 monolayers promising candidates for optoelectronics, photocatalysis, flexible nanodevices and electromechanical systems.