It is pretty difficult to design the piles diameter because of the existing of collapsible loess based on reliability theory.This paper will give the formula of reliability index for vertical capacity consider collapsible.A target reliability index of 3.2 is presented as the criterion for vertical the bearing capacity of pile according to worldwide specifications and references.Based on target reliability of piles,the effective pile diameter is calculated,and the pile spacing is deduced according to effective pile diameter.Finally,through case study,the designed pile diameter using the presented method is more scientific and reasonable because it considers the influences of various uncertainties on collapsible loess,which has great engineering significance.