In order to measure the small cone-hole with a large aspect ratio, the diffraction measurement technology of cone-hole is studied. Based on the basic principles of light wave transmission and diffraction in geometric optics and physical optics, this paper explores the forming mechanism of light spot with small cone hole with large aspect ratio. By constructing the mathematical model of laser diffraction with small cone hole with aspect ratio, the relationship between the image features of spot and the characteristics of small cone hole is analyzed, and the formula for solving the taper and diameter of small cone hole is established. In the research process, the rationality of establishing the mathematical model was demonstrated by comparing the taper and diameter calculated with the parameters set by the model, and the experimental platform was built to carry out the laser transmission micro-hole test experiment. The light intensity distribution curve was extracted by using the spot image obtained by the shooting, and the taper and diameter values were calculated according to the solution formula of the micro-taper and diameter. The results show that the variation trend of the light intensity curve calculated based on the mathematical model of spot diffraction is basically the same as that of the actual light intensity curve. The measurement errors of the taper and diameter of the tiny cone hole with an aspect ratio of 66.67 are 1.35% and 2.78%, respectively. The measurement results have good accuracy, which verifies the feasibility of the measurement method proposed in the paper.