Abstract: With a CCD camera,the time sequence pictures of a transient flow field can be taken up.In order to analysis of the dynamic parameters of the field,it is necessary to make the pre processing of the pictures.This paper introduces a pre processing method including the portioning sharpening,vicinity average filtering,bit mapping,Hilditch linking.Especially,a branch eliminating is used for obtaining the continuous interference lines pattern.This method is well satisfied with the requirement of automatical data processing.