Abstract: Heat transfer was enhanced and the flow path of pyrolysis products were changed in fixed-bed reactor with internals for improving the yield and quality of tar and pyrolysis gas. In this paper,the effects of coal bed thickness on the pyrolysis behavior were studied in a quartz tube bed reactor by simulating fixed-bed internals reactor. The results showed that the yield of pyrolysis tar decreased while the corresponding light tar below the boiling points of 360 ℃ in- creased as the thickness increased. With an increase in coal bed thickness from 100 mm to 200 mm,the tar yield de- creased by 35. 30% ,from 6. 96% to 4. 50% but the light fraction of tar increased by 22. 90% ,from 62． 50% to 76． 80% . The yields of water and gas rose from 9. 32% and 9. 60% to 10. 70% and 10. 90% respectively,while the char yield decreased slightly. In addition,the gasoline fraction yield rose from 3. 80% to 20. 30% and the kerosene fraction yield increased from 18. 50% to 40. 00% .