Polymer-bonded explosive (PBX) is a mechanical weak link in weapons systems. PBX is susceptible to cracking owing to its brittle nature, which seriously threatens the security and reliability of the weapons system. In this work, a novel strengthening method making use of a carbon fibre reinforced polymer (CFRP) patch was adopted to enhance the mechanical performance of the PBX. Brazilian compression tests were conducted on the unreinforced, locally reinforced, and fully reinforced PBX discs to elucidate the strengthening mechanism of the patch. Digital image correlation was employed to characterise the deformation and failure processes of the discs during the full testing period. The results showed that failure of the PBX was caused by initiation and extension of the deformation that localises within the regions near the loading positions. The CFRP patches provided the benefits of load transfer and deformation confinement, which alleviated stress concentration in the disc. Consequently, collapse of the patched discs was delayed, and the load-carrying ability was improved. The findings of this study may deepen our understanding of PBX failure behaviours, and further the performance optimisation of weapons systems.