Abstract: Biomass gasification is a potential technology for providing clean gas in rural area. However, high contents of tar and ash in the gas have become serious problems, because it not only sends out offensive odour, but also clogs the gas pipe. A downdraft gasifier was used as gas generator, tar and ash content in the gas were studied. Besides, the gas cleaning efficiency of cleaning devices, including water spraying device and drying device, were also studied. The results showed that reduction zone temperature has a significant influence on tar and ash producing. High temperature (above 650℃) could reduce tar and ash contents (lower than 2000 mg/m3). Besides, using water-spraying and drying devices, efficiency of gas cleaning devices reached 99.5%, tar and ash contents after cleaning were below 10 mg/m3, accord the gas cleaning demand.