With smoothed particle hydrodynamics, we calculate the formation of micro-jet from the groove of metal surface, and reveal the dependence on the width of loading wave front. The calculated results show that, with the decreasing loading speed,both the mass and the front velocity of ejection decrease, and the mass fraction with low velocity increases. We draw the conclusion that the formation of micro-jet is a result of the particles near the groove acquiring axial velocity and impacting at the axis, and the efflux derives from a layer near the groove. The thin layer becomes smaller when widening the wave front, which is because some particles can no longer satisfy the lock condition of jet strength with the decreasing impact velocity and the increasing impact angle, and then can jet no more.