High precision refractivity of optical glass is an important condition to ensure the image quality, mainly by the V- prism refractometer detection. In V- prism refractometer in using image alignment, the image quality of single line in the collimator directly affects the alignment accuracy in measurement, especially as the contrast between single line and background is low, affects the accuracy in measurement. A combination of selfadaptive gray equilibrium and vertical projection is proposed. This method can extract the low contrast single line, prove the effectiveness of the algorithm through the comparative experiments, and improve the angle measurement accuracy to ± 1″, repeatability superior to 1 × 10- 6, and algorithm stability is good. It has important practical significance to achieve high precision measurement of optical glass refractive index.