The　defects　formed　in　the　manufacture　of　the　vermicular　graphite　cast　iron　engine　cylinder　head　seriously　affect　the　operation　of　the　engine,　which　is　necessary　to　detect.　Ultrasonic　testing　is　a　non-destructive　testing　method　that　has　the　advantages　of　quick　response,　high　resolution,　and　high　security.　In　this　paper,　various　types　of　specimens　are　prepared　corresponding　to　different　types　of　actual　defects　in　the　vermicular　iron　cylinder　head.　An　ultrasonic　A-scan　system　was　built　to　test　the　specimens.　The　short-time　Fourier　transform,　the　continuous　wavelet　transform,　the　empirical　wavelet　transform,　and　the　empirical　modal　decomposition　were　adopted　to　transform　the　signals　into　spectrograms　which　were　further　analyzed　to　reveal　the　inherent　features　of　defects.　The　results　show　that　the　short-time　Fourier　transform　can　be　used　to　distinguish　all　the　common　defects　comparing　to　other　methods.　Comparing　to　the　time-domain　waveforms,　the　transformed　spectrograms　provide　clear　time-frequency　distribution　and　highlight　the　inherent　characteristics　of　the　signal.　©　Published　under　licence　by　IOP　Publishing　Ltd.