The improvement of mechanical properties of granite residual soil is of great significance for the corresponding engineering disaster prevention and control. In order to investigate the improvement effect of low environmental impact biopolymers on the mechanical properties of granite residual soil, xanthan gum (XG), guar gum (GG) and its composite gum (G-X) were used to modify granite residual soil, and the effects of type of biopolymer, mixing amount, maintenance mode, and maintenance age on the improvement effect and their reinforcement mechanism were investigated through the tests of unconfined compression, tri-axial shear and scanning electron microscopy. The results show that XG, GG, G-X can improve the compressive strength of granite residual soil, the compressive strength of improved soil under standard condition increases continuously with the polymer dosage but the increase slows down, while the compressive strength of improved soil under room temperature maintenance will have a peak, and the optimal dosage of polymer is about 1.0%~1.5%; Composite G-X improved soil has better compressive strength (up to more than 2 times of unimproved soil) and age stability, and the shear resistance is obviously improved (the cohesive force is about 2.4 times of unimproved soil), which combines the bonding effect of GG and the filling effect of XG, and reflects the improvement effect of “synergistic efficiency”.