In this paper we investigate the quantum jump in a multi-level atom and construct a formal solution to the optical Block equation describing the interaction system of the multi-level atom with incident resonant laser. It is proved that pure state analysis theory is no longer applicable to a multi-level atom system, so a mixed one is developed, concluding that the resetting density of a multi-level atom should be described by mixed state analysis theory. We also discuss the continuous quantum measurement along with quantum jumping in a multi-level atom.