The invention provides a construction method for improving shield soil in a composite sand gravel stratum. Water, foam and bentonite slurry are used as composite improvement additives; and different additives are selectively injected to the front of a cutter head, the inside of a soil barn or a screw conveyor according to the actual working condition during shield driving with the aid of special improvement equipment and processes so as to play a role in improving the plastic fluidity of the soil, reducing the torque of the cutter head, stabilizing the excavation face and improving the driving efficiency.