Abstract: In the automatic ultrasonic testing system, strong electromagnetic interference is caused by the motor drivers. After hardware-based suppressive methods have been taken, some noises are still induced in the testing signals. The motor interferences were analyzed on the testing system through the signal filtering processing based on software. The influences on ultrasonic testing signals produced by MSMA042A1H AC Servo motor and 110BF003 Stepping motor were compared. After the probabilities of different interferences were calculated and the interference characteristics in time and frequency domains were analyzed, corresponding filter processing methods were brought forward, which provided a gist of motor selection for the ultrasonic automatic testing system.