T-shaped mixing pile is a new technique of composite foundation descend from common mixing pile by expanding the diameter of its upper pile body so as to form a kind of T-shaped mixing pile,which technique has been generally accepted by engineeing world of ground treatment in our country. But the relative design parameters are mainly depend on experience and have not credible basis of theory in practical engineering. This paper analyzed the relation between expanding head diameter of T-shaped mixing pile and its bearing mechanism by the data from field static load test. According to the parameters such as saxial force, additional stress, collapse value of axial force of variable cross section and the ratio of concentration stress and pile top stress which were gotten from test and combined the test curve, it got a reasionable suggestion value of expanding head di ameter of T-shaped mixing pile which was one meter so as to provide reference for design and construction of the method.