The critical length of a pile is an important parameter in a composite foundation. How to economically and reasonably design the composite foundation with piles is a notable problem. Several factors influence the critical length of the pile. How to consider the effect degree of factors on the critical length is a problem to be solved. The critical length of a dry jet mixing pile in a composite foundation of some high-speed railway was analysed through field tests. The factors affecting the critical length of the dry jet mixing pile in the composite foundation were studied by using conventional formulae. The research results indicated that: the critical length of the pile obtained by the formula derived on the concept of bearing capacity and settlement of soil is rather small, and that calculated results from formula deduced on load transmission method is reasonable ； the module of pile, the module of soil, and the displacement of side friction of pile have great influence on critical length of pile, but the Poisson ' s ratio of soil has a little influence on critical length of pile. The new viewpoint of critical length of pile is put forward.