Abstract: With the development of the pile foundation, the cast-in-place piles have been widely used in construction engineering. But the researches on the principles of pile bearing capacity, especially the influence of pile tip soil on the bearing capacity, is still not complete. Based on the data of model tests and field tests on the cast-in-place piles, the influence of different pile tip soil on friction resistance is analyzed, and it is revealed that the increased friction resistance near pile tip is a common situation. In addition, the mechanical properties play an important role in the influence. Finally, by introducing some theories of the strengthened friction the common influence of pile tip soil on friction resistance is proved by test data and theories.