Based on cavity expansion theory, a method for calculating compression modulus of sand compaction pile is developed by considering the change of sand pile volume in the compaction process. The yield zone of the soil around the pile is calculated by Mohr-Coulomb yield criterion. Using the e-p soil compression curve, a formula for average compression modulus is deduced. A method considering the influence of buried depth is proposed for calculating pile-soil stress ratio of sand compaction pile. The Honai-Haiphong highway project in Vietnam is selected as a case study. Analysis indicates that as the casing pipe diameter increases from 0.5 m to 0.6 m, the compression modulus of sand compaction piles and soil around pile decrease, and the average pile-soil stress ratio increases from 2.56 to 2.72. This confirms the statement in Technical Code for Composite Foundation that the pile-soil stress ratio adopts the low value for the soil around pile with high strength, or vice versa. The theoretical calculation result of the pile-soil stress ratio is between 2 and 3 suggested by specification, demonstrating the theoretical method is reasonable and reliable, and greatly improves the sand compaction pile design.