The　receiving　potential　of　high-order　harmonic　signals　for　a　low　frequency　ultrasonic　transducer　is　studied　and　its　usability　for　nondestructive　test　is　analyzed.　The　research　indicates　that　a　normal　transducer　can　receive　high-order　harmonic　signals　with　the　frequencies　of　odd　times　of　fundamental　frequency;　and　the　signal　outside　the　transducer　bandwidth　is　useful　in　ultrasonic　testing.　The　high　order　harmonic　signals　are　weak　and　usually　submerged　in　the　fundamental　signal.　Digital　signal　processing　method　is　used　to　extract　high　order　harmonic　signals,　which　can　be　used　to　test　the　small　flaw　in　spot　welding　and　steel　sheet　structure.　The　application　of　high　order　harmonic　signals　of　transducer　can　not　only　improve　the　test　ability　and　accuracy　of　ultrasonic　transducer　but　also　reduce　test　cost.