We have systematically studied the z-strain effect on electronic properties of monolayer MoS2, MoSe2 and MoSSe through the first-principles calculations. It is found the band-gap linearly decrease with the increase of both tensile and compressive for the monolayers. Due to the energy shift of CBM and VMB in band-structure, the band-gap experiences a semiconductor-metal transition under z-axial strains. And, the modulation of the interaction between the atoms is the main reason for the shift of VBM or CBM. The results have demonstrated that the MoS2, MoSe2 and MoSSe monolayers are promising materials for electronic and optoelectronic devices.