•The MnOx particles assembled with nanosheets were uniformly coated on PET fibers.•The growth process of MnOx layer on PET is clearly clarified.•MnOx/PET showed good activity for HCHO decomposition at room temperature.•MnOx/PET material is promising for indoor air purification due to its light, flexible and low air-resistant properties. Removal of low-level formaldehyde (HCHO) is of great interest for indoor air quality improvement. Supported materials especially those with low air pressure drop are of necessity for air purification. Manganese oxides (MnOx) was in situ deposited on the surface of fibers of a non-woven fabric made of polyethylene terephthalate (PET). As-synthesized MnOx/PET were characterized by SEM, XRD, TEM, ATR-FTIR and XPS analysis. The growth of MnOx layer on PET is thought to start with partial hydrolysis of PET, followed by surface oxidation by KMnO4 and then surface-deposition of MnOx particles from the bulk phase. The MnOx particles assembled with nanosheets were uniformly coated on the PET fibers. MnOx/PET showed good activity for HCHO decomposition at room temperature which followed the Mars–van Krevelen mechanism. The removal of HCHO was kept over 94% after 10h continuous reaction under the conditions of inlet HCHO concentration ∼0.6mg/m3, space velocity ∼17,000h−1 and relative humidity∼50%. This research provides a facile method to deposit active MnOx onto polymers with low air resistance, and composite MnOx/PET material is promising for indoor air purification.