Defects are inevitable in the synthesis of monolayer transition metal dichalcogenides （TMDs） and will impose important influence on the physical and chemical properties of monolayer TMDs. In order to study the influence on the piezoelectric effect of monolayer TMDs with grain boundaries （GBs）， the piezoelectric coefficients of 36 kinds of monolayer TMDs with GBs and 36 kinds of perfect monolayer TMDs are calculated based on density functional theory （DFT）. The results show that the GBs enhance the piezoelectric effect of monolayer TMDs. This is owing to that GBs lead to strain gradients and the flexoelectricity is triggered. The change in piezoelectric coefficients shows an obvious periodic trend. The piezoelectric coefficient increases gradually with the relative atomic mass of chalcogenide elements increase. The maximum piezoelectric coefficient of monolayer MoTe₂ with GBs is 11.17 pm/V， which is enhanced about 27.8% compared with that of the perfect MoTe₂ （8.74 pm/V）. These results provide a theoretical guidance for the development of high-sensitivity sensors and high-precision controllers in aircraft.