The laser welding test was carried out on the 5083 aluminum alloy with thickness 1.0 mm and the 6016 aluminum alloy with thickness 1.2 mm. The relationship between plasma MgI spectrum signal and the process parameters as well as welding quality were studied by testing the MgI singal of laser welded dissimilar alloy and by analyzing fracture morphology,microhardness and mechanical properties in the joint by using spectroscopic measurement system, scanning electron microscope, vickers microhardness machine, and computer control electronic universal testing machine. The results show that, with the speed increasing, MgI spectrum signal strength is reduced; the focus obviously affects MgI signal while the laser power changs the whole MgI signal amplitude slightly. When MgI relative strength appears smaller or larger, the specimens have lack or over welding; MgI plasma with strong fluctuations leads to defects such as porosity and low mechanical performance while smooth MgI signal has less defect and good mechanical performance. The real-time control welding process X -Rs is established based on MgI spectral signal corresponding to the quality of weldment and Hugh hart SPC control theory.