All-direction unequal legs transmission tower are widely applied on the transmission projects in mountainous areas while long time-consuming of composition calculation is always the greatest obstacle to accurate optimal analysis. An improved substructure method is introduced in the calculation prcocess of internal force due to comparability of different assembled models for unequal legs. The transmission tower is divided into two parts including the substructure and the leg stucture. The block decomposition algorithm is used in the calculation of equivalent stiffness matrix and load matrix. As a result, the DOF condensation and border nodal treatment are accomplished, so that the time is saved for most of composition and decomposition of DOF and the calculative efficiency enhanced. Furthermore, an idea is put forward for reducing the calculation capacity by combining with time consuming analysis.