Abstract: A large number of experiments indicate that the m value of piles under lateral load decreases with the increase of soil surface displacement.However,the existing codes haven’t provided the selecting range and methods under large displacement condition,resulting in difficulty in engineering design.Based on the measured data of piles under lateral load,mathematical statistics are used to establish a relationship between the m value and its soil surface displacement under various soil conditions,and it can be used to calculate the m value under large displacement.The example demonstrates that the present method is of reliable precision and practicability.