Duckweed (DW) has a promising potential for wastewater treatment due to its outstanding performance in the fixation of nutrient elements and heavy metals. The conversion of harvested duckweed into value-added products through pyrolysis is an attractive method for duckweed utilization as fuels or chemicals. In this work, the duckweed was prepared by deashing treatment and subsequent impregnation with different phosphoric acid concentrations (ADW-P). The pyrolysis behavior and kinetics of raw and impregnated duckweeds were studied with respect to the ash contained in the duckweed and the phosphoric acid catalytic effect by thermogravimetric analysis-fourier transform infrared spectrometer (TG-FTIR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The results show that the pyrolysis reaction zone is extended for the impregnated duckweed by reducing the initial pyrolysis temperature and increasing the final temperature. Specifically, a shoulder peak situated at the right side of the main peak is present in the DTG curves of ADW-P, suggesting the formation of relatively stable substance after phosphoric acid impregnation. Phosphoric acid promotes the release of H2O, CH4, CO2 and NH3 as well as light volatiles with C[dbnd]C and C[dbnd]O groups during pyrolysis. Meanwhile, decarboxylation and aromatization as well as deamination of organic compounds are strengthened, resulting in an increase of aromatic hydrocarbons and furans and a decrease of N-heterocyclic compounds. The activation energy of ADW-P is lower than that of DW and ADW at conversion rate less than 75% but increases remarkably at high conversion rate. This implies that phosphoric acid facilitates the thermal decomposition of DW at low temperatures but hinders its decomposition at high temperatures probably due to the formation of stable cross-linked structures such as phosphate and polyphosphate esters.