The combined pile composite foundation has the advantages of good technical and economic effects, yet there are no mature analysis methods to practise in design and test at present. This paper has studied the three and four-element composite foundation by the plate loading test, in which the pressure cells, pressure manometers and sensors are laid on the top of the piles and soil. The results show that the stress on the top of piles and soil increases with the increase of total load, the increasing rate being directly proportional to the strength or relative rigidity of piles and soil. The various increasing rates of piles and soil stress versus load make great difference between the three and four-element composite foundation in the variation of the load sharing ratio and the stress ratio versus load. The settlement of the combined piles composite foundation varies gradually with pressure, and the p-s curve may be simulated by hyperbola curve so as to establish the failure criterion and obtain tangent modulus. As the pressure increases, the inter-pile soil enters plastic state earlier than the piles, so the latter have to bear majority of the load, which makes the settlement rise. The composite foundation should be designed according to settlement-control principle.