In view of the collapse of the trench wall and the failure of conventional plugging materials such as cement in the construction of diaphragm walls in water-rich sand and gravel formations， single factor experiment was used to analyze the influence of the gel forming agent （anionic polyacrylamide）， the crosslinking agent （chromium crosslinking agent）， the reinforcing agent and clay on the properties of polymer gel （gelation time， viscosity， strength and stability） for shallow plugging； and the optimum formulation was obtained by orthogonal test. In addition， the formation mechanism of the gel material was also analyzed. The results show that the reinforcing agent can link the gel-forming agent in gel， and further enhance the strength and stability of gel. Clay can also shorten the cross-linking time and enhance the gel strength， but clay is not conducive to the stability of the slurry. The final optimized formula is： 15% chromium crosslinking agent+5% gelling agent+8% reinforcing agent+7.5% clay. Its performance can meet the plugging requirements of shallow diaphragm walls.