The　higher　order　harmonic　component　of　nonlinear　ultrasonic　is　obtained　by　the　general　ultrasonic　testing　device　with　a　higher　pass　filter,　and　the　higher　order　harmonic　wave　is　adopted　into　the　tiny　flaw　detection　and　the　ultrasonic　imaging　of　spot　welds.　The　higher　order　harmonic　wave　has　excellent　sensitivity　to　the　tiny　flaw　and　higher　resolution　in　the　time　domain　than　conventional　ultrasonic　wave.　Thus,　the　size　and　the　position　of　tiny　flaw　are　easier　to　determine,　though　the　thickness　of　spot　weld　is　very　small.　Besides,　the　nonlinear　ultrasonic　imaging　technique　based　on　the　higher　order　harmonic　wave　is　also　presented　in　the　research,　and　the　nonlinear　ultrasonic　imaging　is　more　effective　to　investigate　the　inner　constitution　of　spot　weld.　The　nonlinear　ultrasonic　has　a　higher　testing　effectiveness,　and　the　testing　ability　of　ultrasonic　system　can　be　developed.　©　Right.