A self-adaptive background forecast algorithm was presented aiming at image identification under complex background in magnesium alloy melt.Through judging whether the gray difference of the current pels’s neighbor pels is more than the threshold valve preset

different background forecast algorithms were self-adaptively selected.If the gray difference is more than threshold value

blocking weight background forecast algorithm is adopted

while it is less than the threshold value

fixed weight background forecast algorithm is selected.Furthermore

the self-adaptive background forecast algorithm was compared with the fixed weight background forecast algorithm and blocking weight background forecast algorithm.The results show that the algorithm not only can reflect the real size of the first bubble better

but also can effectively improve the PNSR（peak signal and noise ratio） and calculating speed.