In this paper, endoscope with aided lens was designed for observation of fire-fighting foam. Firstly, water-proof and corrosion-resistant endoscope was completed according to the foam’s characteristics. Then, aided lens for clear observation was designed corresponding to the endoscope’s lens system. Secondly, the fire-fighting foam was produced using low expansion foam extinguishing system and observed through endoscope with aided lens. Finally, the data were analyzed considering the foam’s drainage process. This endoscope with aided lens could be used to describe foam’s interior structure’s evolution including foam drainage, film rupture and bubble coarsening.