Abstract: The utilization of duckweed in wastewater treatment has gained ever-increasing attention in recent years. Meanwhile, the harvested duckweed can offer the potential to use for energy production. As a promising thermochemical conversion technology, the pyrolysis can convert the solid fuel to bio-char, bio-oil and combustible gas products with high efficiency. However, the duckweed generally has a high ash content, such as alkali, alkali earth metals, and minerals, due to the special cultivation environment and harvesting. This has posed a great challenge for the thermochemical conversion and the quality of pyrolysis products. Therefore pretreatment, such as acid washing, is necessary to reduce the content of these minerals in the conversion process. In this work, a pyrolysis experiment was performed on the duckweed, which was normally used to mitigate the water pollutions in Erhai Lake, Yunnan Province, China. Acid-washing was firstly conducted for the duckweed to remove the inorganic species in the fuel. Then, the pyrolysis experiments of the raw and acid-washed duckweed were carried out in a fixed bed reactor at 400-900 ℃, with an interval of 100 ℃, to investigate the effect of acid-washing on the yield and composition of solid, liquid and gaseous products. The results showed that the acid treatment can effectively remove most of the alkali, alkaline earth metals, and the majority of phosphorus in the biomass. Meanwhile, the fixed carbon and C content were greatly improved. The heating value increased substantially to 19.4 kJ/kg, which was close comparatively to the woody biomasses with a heating value of 17.6-20.8 kJ/kg. The results of pyrolysis experiments showed that the solid products decreased but the gas products increased with the increase of temperature in both raw and acid-washed fuel. Conversely, the liquid products increased firstly to the maximum at 500 ℃ and decreased with the increase of temperature. After treatment, the solid and gas products decreased by 1.43%-8.02%, and 2.81%-19.89%, respectively, with the increase of liquid products by 1.63%-16.72%. In acid-washed duckweed, the solid, liquid and gas products changed slightly, when the pyrolysis temperature was higher than 600 ℃. In addition, there were more significant differences of liquid and solid products between the raw and acid-washed duckweed at 700-900℃. In the gas products, the acid treatment relatively reduced the release of CO, CO2, CH4, and H2. However, the reduction of CO2 was much higher than that of the other gases, and thereby the heating value of gas products increased by 5%-155%, compared with that from the raw duckweed. In the solid products, both the fixed carbon and C content of the char samples from the acid-washed duckweed increased significantly with the decrease of ash and O content, compared with that from the raw duckweed. The composition of bio-oil changed slightly after acid treatment. Specifically, there was a slight change trend of N-containing species, aliphatic hydrocarbon, and aromatic hydrocarbon after acid treatment. However, the content of phenols, furans and carbonyl compounds in bio-oil decreased with the significant increase of alcohols and ether at 400-600 ℃. The acid treatment can greatly improve the quality of gas and solid products, where the content of combustible compounds increased for the high heating value. In the liquid products, both the raw and acid-washed duckweed showed potential utilization in producing N-containing species and aromatic hydrocarbon at low and high temperatures.