The work aims to study the influence of the light intensity change of the LED backlight source on the accuracy of the two-dimensional size measurement of the visual measurement system. Through carrying out the theoretical derivation and experimental study of the relationship between image gray scale and light intensity, the influence rule of the backlight intensity on the edge position of the image was obtained. With the enhancement of the backlight intensity, the pixel gray value near the edge position gradually increased and edge position shrank up to 4.7 pixels (20.03 μm). The measurement error of the diameter of each standard axis was reduced to within 1 pixel (4.26 μm) in the method of the dynamic edge position extraction with adaptive light intensity change. The light intensity change has great influence on the measurement accuracy of the image measuring system. The method of dynamic edge position extraction with adaptive light intensity change can effectively compensate the measurement error and improve the measurement accuracy.