The nick depth of easy open end is a key parameter in ensuring high packaging quality. In order to realize automatic detection of easy open end in industrial production sites and improve the measurement accuracy and stability, a machine vision system based on light- section method is developed, which has a three dimensional positioning function and an image measuring function.In view of the light- sectioning image properities, the geometrical relationship between the measuring position and the notch slope in the x-y plane is analyzed through experiments to offer a two dimensional location strategy for automatically detecting and adjusting the light projection position. Through analyzing six focusing functions for the light-sectioning images, unimodal and unbiased as evaluation standard an automatic focusing algorithm based on the sum of gray rate of change function as rough focusing function and Laplacian function as fine focusing function is proposed to adjust the notch location. Experimental results show that the proposed algorithm has the strong single peak feature, good stability and high sensitivity.