Abstract: In order to thoroughly study the spatial strut-and-tie theory on the punching failure of pile caps, two critical problems, value about end cross dimension of strut and calculation on strut strength, have been carried out based on the literatures and the research results from author. The results indicate that the end cross area of strut should be smaller than the cross area of pile, that there are difference on the value and dimension number of formula variables between a two-dimensional strut strength and a three-dimensional strut strength, the formula variables of the three-dimensional strut strength should be the ratio of punching-span and concrete strength.