A novel method for the direct detection of gaseous formaldehyde based on the fluorescence quenching of CdTe quantum dots (QDs)/polyelectrolyte multilayer films (QDMF) was proposed in this paper. The functional multilayer films were assembled by layer-by-layer (LBL) deposition of oppositely charged CdTe QDs and poly(dimethyldiallylemmonium chloride) (PDDA). Formaldehyde can quench the fluorescence of CdTe QDs multilayer films effectively. Under the optimization conditions, the fluorescence intensity of QDs decreased linearly with the increase of formaldehyde concentration in the range of 5-500ppb. The detection limit for formaldehyde was 1ppb. The quenching mechanism of CdTe QDs multilayer films by formaldehyde was also studied in detail. This proposed approach was simple, rapid and had excellent selectivity and sensitivity for the detection of gaseous formaldehyde.