The invention relates to a construction method for composite foundations.The construction method comprises the steps that firstly, steel pipe sets composed of a plurality of minor-diameter steel pipes aim at pile positions to be sunk into a soil body to a set depth; secondly, the steel pipe sets are elevated while fine concrete is pumped into the minor-diameter steel pipes; thirdly, operation in the second step continues to be conducted until the steel pipe sets are elevated out of the ground, and a set of minor-diameter piles is formed in the foundations; fourthly, operation from the first step to the third step is repeatedly conducted until the foundations of a whole needed field are full of the minor-diameter piles; fifthly, a mattress layer is laid on the tops of the minor-diameter piles, and the composite foundations are formed.According to the construction method, the pile soil displacement area is increased by reducing the pile diameter and increasing the number of the piles, and hole forming, hole extruding and pressing filling are finished at a time; multiple piles can be constructed at a time, the structure is simple, continuity is high, and work efficiency is high.