The invention discloses a high polymer material slurry filled discrete material pile composite foundation and a construction method thereof. A discrete material pile is filled in a soft soil foundation through a vibrating immersed pipe, a rigid injection pipe is embedded in advance in the middle of the discrete material pile, a barrel-shaped cover plate is embedded on the top of the discrete material pile, and high polymer material slurry is sprayed to the discrete material pile at high pressure through a slurry injection system. The high polymer material slurry is filled in the clearance of the discrete material pile and splits to permeate into pile soil, so that the discrete material pile body is engaged and contacted with the soil around the discrete material pile, and the friction coefficient of the pile-soil contact surface is effectively increased. Bar pile caps are poured on the top which is formed by a high polymer material slurry filled discrete material pile body, a high polymer material slurry filled soil body and the pile soil. A high polymer material is insoluble and has certain expansibility. In the process of spraying at high pressure, water in a part of the soil body is discharged through a passage of the discrete material pile. The high polymer material slurry filled discrete material pile composite foundation has the advantages of simple construction process, strong operability, convenience in quality control, good engagement effect of the pile-soil contact surface, obvious economic benefit and significant effect for reinforcing the soft soil foundation.