The invention discloses a control method for support pressure of an excavation face of a shield tunneling machine in a sand-pebble layer, and belongs to the technical field of shield tunneling machines. The control method comprises the following steps: S1, filling compressed air into a pressure chamber of the shield tunneling machine to keep pressure in the pressure cabin stable; and S2, sprayingmodified mud to the excavation face through a cutter head. The control method for support pressure of the excavation face is suitable for a condition with muck and compressed air in the pressure cabin; the compressed air is filled into the pressure chamber, so that pressure in the pressure chamber is kept stable, and therefore, stability of the excavation face is ensured; modified mud is sprayed to the excavation face, and the modified mud penetrates to the excavation face under action of the compressed air, so that a mud film is quickly formed on the excavation face, escape of air is reduced,and support pressure of the excavation face is stabilized; modified mud is injected into the pressure chamber, and muck containing sandy cobbles is improved to be a plastic flow state, so that a spiral mud discharger smoothly discharges muck, and the muck is also prevented from flowing under action of the compressed air.