[Objective] Taking Yanhe River basin as a study area, typical loess soil line equation was established by automatic algorithm to provide basic parameters for the calculation of the soil adjusted vegetation index. [Methods] Based on Landsat 8 OLI images, soil line was extracted by automatic algorithm, then the correlation coefficients between normalized difference vegetation index(NDVI), perpendicular vegetation index(PVI), transformed soil-adjusted vegetation index(TSAVI), adjusted transformed soil-adjusted vegetation index(ATSAVI) and test coverage were calculated; and the applicability of soil line equation in vegetation index extraction was also discussed in the Loess Plateau. [Results] (1) Compared with conventional method, the difference was small, and the automatic algorithm had high accuracy and stability; (2) The sequence of the correlation between vegetation index and test coverage was: PVI> NDVI> TSAVI> ATSAVI, which showed that PVI was the optimal vegetation index, NDVI was the next, TSAVI and ATSAVI were the worst to the extraction of vegetation coverage in Yanhe river basin; (3) Compared with NDVI, PVI index could reduce the influence of soil noise, and its sensitivity to different vegetation types was higher. PVI was more suitable for the Loess Plateau with low vegetation coverage. [Conclusion] It is suitable to extract the soil line by automatic extraction algorithm in Yanhe river basin, and the parameters can be used to calculate the soil-adjusted vegetation index.