Composite T-beam in general use is composed of concrete and steel. Compared with mechanical analysis, little research has been carried on deriving the theoretical model for back analysis of composite T-beam. Firstly for composite T-beam, degraded solid element theory is deduced and the displacement function is obtained. Then Markov error function of mechanical parameters of composite T-beam is founded. The back analysis computing formulas are also obtained by adopting optimization method including revised Powell method. Then the steps of back analysis of mechanical parameters of the composite T-beam are given. Through analysis of a classic example, some important conclusions about revised Powell back analysis of mechanical parameters of composite T-beam are obtained.