Abstract: There are more and more deep foundation pits of subway in Lanzhou area. In order to protect the ecological environment and reduce the project cost, after excavation, adding the cement and loess into the red sandstone is used as the roadbed filler through experiments. Based on a subway project in Lanzhou, the compaction tests and the quick shear tests are carried out on the remolded soil with different mixing ratios by adopting orthogonal tests. The influence factors of the shear strength of the red sandstone-improved soil are analyzed, and the best mix proportion of each factor is given. The regression model for shear strength of red sandstone-improved soil is obtained. The test results indicate that the loess has significant influence on the internal friction angle, and the water content and cement have a significant influence on the cohesion. The order of main influence factors of cohesive force can be arranged as follows: cement→water content→loess, and the order of main influence factors of internal friction angle can be arranged as follows: loess→water content→cement. According to the test data of different mix proportions, the regression equation is established to predict the cohesion and internal friction angle of the improved soils. The results have certain reference value for the evaluation of red sandstone as the backfill material.