To better evaluate the internal friction angle of shallow cushion layers in pile-soil composite subgrades, the relationship between the internal friction angle of sand cushion layers and the specific penetration resistance was analyzed based on cavity expansion theory and static cone penetration tests. Laboratory calibration tests were conducted using a probe with an optimized diameter, incorporating Bolton’s stress-dilatancy theory, so as to establish a calibration relationship between the internal friction angle and relative deformation in gravel cushion layers. Field tests were carried out to determine the variation ranges of internal friction angles of shallow cushion layers in different types of composite subgrades. The results show that the specific penetration resistance increases with the increase in the internal friction angle of the sand cushion layer. In the laboratory calibration test, the internal friction angle ranges from 28.3° to 35.1° for sand cushion layers and from 33.1° to 42.6° for gravel cushion layers. Moreover, the average value of the internal friction angle of sand cushion layers in flexible pile composite subgrades generally ranges from 33.5° to 36.0°, while for gravel cushion layers in semi-rigid and rigid pile composite subgrades, the internal friction angle is between 37.0°-39.0° and 38.5°-40.5°, respectively.