The invention discloses a testing system for testing the mechanical property of a plant root-soil complex on a side slope. The testing system comprises an I-shaped test soil pit, a vertical loading group, a horizontal stepwise loading-stress group and a shearing displacemant measuring group, wherein the I-shaped test soil pit is protectively dug around a plant to enable a soil base where the plantis positioned to be at the center; a cob wall is reserved at a part, close to the soil base where the plant is positioned, of the soil pit on one wing of the I-shaped test soil pit; the soil pit is defined as a rear pit, and the soil pit on the opposite wing of the I-shaped test soil pit is defined as a front pit; the vertical loading group comprises a vertical loading plate, three groups of supporting columns and a pile-loading material; the vertical loading plate spans the soil base where the plant is positioned and the ground surfaces on two sides, and a gap for allowing the plant to passthrough is reserved; the three groups of supporting columns are used for supporting the vertical loading plate and are symmetrically arranged on the ground surfaces on two sides and the soil base where the plant is positioned; the pile-loading material is piled on the vertical loading plate right above the soil base where the plant is positioned; the horizontal stepwise loading-stress group comprises multiple layers of shear cells, multiple layers of earth pressure cells, a plurality of backing plates, vertical pull rods and pulleys. The system can test the mechanical property of root-soil complexes with different depths on the side slope.