The structural, electronic, dipole-induced internal electric field, optical and photocatalytic properties of monolayer GeS and GeSe under external biaxial strain were investigated by using first-principles calculations. The monolayer GeS and GeSe are indirect semiconductors with the band gaps of 3.265 eV and 2.993 eV, respectively. The band alignment of the monolayer GeS and GeSe manifests the photocatalytic activity for water splitting. Especially, it is effective to tune the properties including structures, band gaps, surface potential difference, dipole moment P, dipole-induced internal electric field, absorption and photocatalytic activity of the monolayer GeS and GeSe via biaxial strain. Our results suggest that monolayer GeS and GeSe possess photocatalytic properties for water splitting, and strain engineering, especially tensile strain, can enhance the photocatalytic activity under ultraviolet and visible light.