Abstract: In order to better control the grouting area, pressure and quantity during the post grouting at the tip of cast-in-place concrete piles, a composite post grouting technology is proposed. Based on the Xilingjing highway bridge project, the composite post grouting and vertical static load tests on two cast-in-place concrete piles (SZ1 and SZ2) are performed to study dynamic response laws of grouting pressure, grouting quantity and pile-top displacement during the grouting process as well as their bearing deformation characteristics. Based on the theory of spherical cavity expansion and the generalized SMP criterion, the analytical formula for the radius of grouting bubble at pile tip is derived, and the method for calculating the load-settlement relationship of composite post grouting cast-in-place piles is established. The research shows that the composite post grouting process of "open-close-open'' at the pile tip is an alternating process of splitting, penetration and compaction, the grouting quantity reaches the design value, the grouting pressure is stable, and the lifting displacement of the pile top is small. The strengthening effects of the lateral friction of SZ1 and SZ2 within the range of 2.5 times of pile diameter of pile tip are significant when the strengthening coefficients are 3.37 and 3.45 respectively, and the strengthening coefficients of tip resistance are 3.28 and 5.37 respectively. The load-settlement method based on the load transfer model of hyperbolic function and considering the enlarged head effects of pile tip can accurately predict the load-settlement relationship of composite post grouting cast-in-place piles.