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In practical Lamb wave-based damage detection, the spatial resolution of Lamb waves is usually restricted by not only dispersion but also the space duration of excitation waveforms, i.e., the initial spatial resolution (ISR) for the signals before travelling. To overcome this problem, a signal domain transform (SDT) method is presented in this paper. In SDT, the original dispersive Lamb wave signals are transformed from the time to distance domains, with the time-distance scaling on the excitation waveforms particularly modified. Then, both dispersion compensation and ISR enhancement can be actualized to efficiently improve the signal spatial resolution. A high resolution damage imaging based on SDT is further developed. The efficiency of SDT and SDT-based imaging methods has been well demonstrated by the experimental study on an actual complicated aircraft wing beam.