Mineral quantitative analysis by optical microscope is simple and reliable, and has been widely used in process mineralogy and rock-mineral determination. The line measurement method is fast and efficient, and is suitable for fine-grained mineral particles, which plays an important role in the application. However, the conventional test line has the characteristics of directional identity, and cannot deal with the problem of 'preferential orientation' of the analyzed mineral particles. To solve this problem, a new mineral quantitative method by rotating the platform of an optical microscope was conducted. The main principle is that the length ratio of the circular cutting lines of different mineral particles is equal to their volume ratio. The main test process of the method is as follows:rotating the objective table for a circle and testing the angles (ΔK
) that the circular cutting line contained, which is intersected by the mineral particles and the visual field under the microscope. By testing a certain number of visual fields (N
) and calculating weight percentage (W
), the percent content is acquired by the equation W
)×100%. After theoretical analysis, the galena, sphalerite, and magnetite thin sections were examined. The results show that the new mineral quantitative method is simple, accurate, and the difference value of the test results is less than ±5% compared with the conventional line measurement method. In addition, this method can be used to effectively solve the negative effect of 'preferential orientation' on the test results.