Piezoelectricity can realize the mutual conversion between mechanical energy and electrical energy. With the development of science and technology, the biggest challenge now is to look for the materials with higher efficiency piezoelectric effect. Using first-principles calculations, we study the elastic and piezoelectric properties of the V-IV-III-VI sheets: SiN-AlO, GeN-GaO, and SnN-InO. The Young moduli of these three V-IV-III-VI sheets are much smaller than graphene. Among these three V-IV-III-VI sheets, the in-plane piezoelectric coefficient d(11) (12.69 pm/V) of SnN-InO is the largest, which is larger than that (3.73 pm/V) of molybdenum disulfide (MoS2) by 3 times. SnN-InO also exhibits the largest out-of-plane piezoelectric coefficient d(31) (8.66 pm/V), which is larger than that (0.46 pm/V) of the Janus group-III chalcogenide sheets by 18 times. The broken inversion symmetry and the large electronegativity difference between the atomic top layer and bottom layer of the XN-MO sheet induces the out-of-plane dipolar polarization, leading the out-of-plane piezoelectric effect. The flexible SnN-InO with remarkable out-of-plane piezoelectric properties can be applied in nanoscale energy harvesting devices and nano-sensor in medical devices.