When a transformer is switched back into power system operation again without load from last switching off， due to either the action of protective device aiming at the system fault or the changes of system framework structure， the residual flux of ferromagnetic-core is non-random. Based on the time-characteristic of ferromagnetic material analysis， combined with Preisach model， a way to calculate residual flux is proposed. An expression between the residual flux and the entering-phase is derived to eliminate inrush current in single-phase transformer and its correctness is shown by an EMTP test. Meanwhile， some rules between the residual flux and the range of entering-phase at a given overshoot limit of inrush current are demonstrated in the simulations. Finally， these rules have been used to propose a control strategy about entering-phase in sections to design a device to eliminate the inrush current.