A distorted low-symmetry Eu4L4 tetrahedral cage was fabricated through the self-assembly of europium ions and C3-symmetric bowl-shaped ligands containing a phosphangulene core. X-ray crystallography confirmed its unique architecture, featuring significant structural distortion and solvent-accessible coordination sites. This cage showcases exceptional luminescence-based sensitivity for formaldehyde detection compared to other analogs, achieving an impressive detection limit of 19.4 ppb.