The invention relates to a residual voltage estimate based seamless switching power supply method for a motor. A system adopted in the method comprises a working power supply and a standby power supply which are connected with the motor respectively, wherein a rectifier is connected between outputs of the working power supply and the standby power supply and is further connected with the motor through an inverter. The method comprises steps as follows: S1, modeling of residual voltage of an induction motor after power loss; S2, prediction of residual voltage of a bus of the motor; S3, predication of the initial value of the residual voltage of the bus of the induction motor after power loss: when the initial value of the residual voltage of the bus is smaller than rated voltage of the bus, a UPS (uninterrupted power supply) inverter is started and stabilized in a work state for supplying power to the induction motor; and S4, the phase of the residual voltage of the bus is compared with the phase of voltage of the standby power supply, and the standby power supply is put into use when the phase difference is smaller than 20 degrees. By means of the method, power can be supplied to an important load during power loss of electric supply, output voltage can be also initiatively adjusted, the standby power supply can be rapidly switched and put into use, impact to the load is avoided, and the instant impact current when the standby power supply is put into use can be effectively limited.