Abstract: This paper expounds four types of poor background parts and lays key point on the process analysis and process improvement through contrast test. It was verified that the main causes of poor background fluorescence were due to the contaminated sand while the blowing sand machine was in processing the parts. Under high pressure, the contaminated sand caused secondary pollution of other products. Before FPI process, the secondary pollution of products was not cleaned off, so large yellow-green areas were presented. In this paper the technological improvement measures are put forward to improve the defect inspection rate and ensure the normal production of enterprises.