The improvement of mechanical properties of granite residual soil is of great significance for engineering disaster prevention and control. In order to investigate the improvement effect of eco-friendly biopolymers on the mechanical properties of granite residual soil, xanthan gum (XG), guar gum (GG) and their composite gum (G-X) were used to modify granite residual soil, and the effects of biopolymer type, content, curing mode, and curing age on the improvement effect and its reinforcement mechanism were investigated through unconfined compression strength, triaxial shear strength and scanning electron microscopy tests. The results show that XG, GG, and G-X can improve the compressive strength of granite residual soil, and the compressive strength of improved soil under standard curing conditions increases continuously with polymer dosage but the increase slows down, while the compressive strength of improved soil under room temperature curing peaks, and the optimal dosage of polymer is about 1.0%-1.5%; The composite G-X improved soil has better compressive strength (up to more than twice that of untreated soil) and age stability, and the shear resistance is obviously improved (cohesion is about 2.4 times that of untreated soil), which combines the bonding effect of GG and the filling effect of XG, and presents a synergistic improvement effect.