The result of pile tests on 14 piles with different lengths in 5 engineering cases shows that bearing capacity of single pile was quite different in the same construction site and with the same type of pile just because of different pile lengths and bearing stratums. The paper analyzed the conditions of bearing capacity of pile tip and time effect, when bearing capacity of pile tip was weak, lower bearing capacity of single friction pile could be reasonably explained by compressive pressure to pile or by A.S.Vesic theory.