Experiments of laser welding of 1.6 mm thick galvanized steel B340/590DPD+Z with different amounts of copper are performed by using a CO2 laser. Based on the light emission from the plasma plume acquired by spectrometer in the welding process, the effects of adding copper powder on the plasma optical signal and the relationship between the pores and the plasma optical signal is analyzed. Then the average content of copper element and the distribution of the main elements around the pores are detected by using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results show that compared to laser welding without adding copper powder, the oscillation of plasma temperature is significantly reduced and the average temperature drops about 5000 K. In addition the solid solution alloy of copper and zinc distributes around the pore wall and the porosity has a correlation with the intensity of spectrum at CuI324.8 nm as well. The researches provide a theoretical basis for monitoring on laser welding galvanized steel with powder feeding and a new method for the plasma control technology.