A　nonlinear　ultrasonic　array　imaging　method　used　for　the　defect　loacation　is　developed.　The　propagation　characteristics　of　the　focusing　acoustic　field　in　time-space　domain　and　the　conditions　to　generate　the　diffusion　acoustic　field　are　investigated,　then　the　relationship　between　the　diffused　acoustic　energy　in　two　different　phased　mode,　parallel　and　sequential　focusing,　and　the　nonlinear　effects　is　analyzed,　and　a　nonlinear　ultrasonic　imaging　method　is　proposed,　in　which　the　difference　of　acoustic　energy　of　two　phased　modes　within　the　excitation　bandwidth(fundamental　wave)is　considered　as　characteristic　parameter　for　imaging.　The　influence　of　time　delay　and　time　window　for　diffuse　field　sound　energy　and　nonlinear　imaging　are　studied,　and　the　optimal　parameters　for　nonlinear　imaging　are　obtained.　The　time　delay　has　great　influence　on　the　nonlinear　imaging,　however　the　width　of　window　has　little　effect　on　the　nonlinear　imaging.　Over　a　range　of　time　delay,　the　diffusion　acoustic　field　is　stable,　therefore　the　micro-crack　can　be　detected　effectively.　It　is　demonstrated　that　the　proposed　nonlinear　imaging　technique　is　an　effective　method　for　micro-crack　detection.　©　2016　Journal　of　Mechanical　Engineering.