Magnesium plays an important role in the hardening mechanism of aluminum alloys, but sensitisation-induced intergranular corrosion cracking limits the widespread use of aluminum alloy in equipment. For on-site quantitative assessment of sensitisation in 5-series aluminum alloys, a laser-induced plasma imaging technique is proposed, which evaluates the degree of aluminum alloy sensitisation by obtaining images of the plasma formed by laser ablation of aluminum alloys, and then classifying and quantifying the images using a residual network. Compared to EMAT, XRD, ECT and LIBS techniques, the sample surface only needs to be polished, does not consume chemical reagents and is not affected by the shape and thickness of the workpiece, which provides higher quantitative accuracy, stability and detection efficiency. It is a more suitable technology for on-site inspection of aluminum alloys. The effects of laser energy, sample surface roughness and image data set on the quantitative results are systematically investigated, in particular the consistency of mixed detection results of sensitised samples at different temperatures. Finally, the method is discussed in terms of the sensitisation mechanism of aluminum alloys and its applicability to the quantitative detection of sensitised 6- and 7-series Aluminum alloys by the detection of H-atom content.