The conventional master-slave-splitting based global power flow calculation method for the whole transmission and distribution networks is effective in the case of radial distribution network. However its convergence often faces problems when there are loops in the distribution networks. In order to deal with such a situation， a modified master-slave-splitting method is proposed by equivalenting distribution networks. A new distributed iteration scheme between transmission and distribution power flows is proposed， in which the loop flows in the looped distribution networks can be approximatively considered in the transmission power flow sub-problem， which results in significant improvement in convergence. The new method is discussed in detail. Numerical tests show that the proposed method has good performance in convergence with less requirement in communication， and is suitable for online distributed computation between transmission and distribution control centers.