A automatic collimating method with vibrating slit for the photoelectric automatic refractometer with high precision was introduced in this paper and geometry size relation of width among image and vibrating slit and amplitude was described also. This collimating method was realized through acquiring really-time signal of monochromatic light from sample measured and judging automatically collimating point on the basis of amplitude and phase of this signal by computer. The circuit principle and constitution were represented in detail. Many optical materials are measured in the wavelength rang of 365 to 2600 nm .The results show the precision of this collimating method is batter than ?0.1".