Abstract: The effect of tempering temperature on the microstructure and mechanical properties of EH40 high strength ship plate steel with thickness of 68 mm was studied by optical microscopy, scanning electron microscopy and mechanical properties detection device. The results indicate that there were little changes in the microscopic structures after tempering from 500 ℃ to 670 ℃ for the steel plate, but with the increase of tempering temperature, the carbides precipitated dispersively and distributed uniformly in steel plate interior, and the strength decreased and toughness increased; the steel plate had optimal strength and ductility properties after tempering at 600 ℃ for 3.5 h.