In the process of starting, the electro motive force(EMF) of doubly salient electrical motor (DSEM) can be maintained for the constant power control by the flux weakening in theory as the flexible flux. However, the nonlinear magnetic characteristics of DSEM cause EMF to be anomalous square wave. It is difficult to control the excitation current by the close loop of EMF as DC motor. In this paper, the excitation current of DSEM is constantly controlled in the process of constant torque. The flux weakening control method with the feed back of the rectified voltage for three terminal phases is given. The switch process from the state of constant torque to constant power is smooth and the maxim output of DSEM is maintained in the process of constant power. Simulation and experimental results confirm the theory of the flux weakening method.