Abstract: In order to grasp elm sawdust pyrolysis behavior, slow pyrolysis of elm sawdust at the heating rate 20℃/min was studied in this paper by utilizing thermal analysis coupled with mass spectrometry technique. The evolution profiles of seven kinds of small-molecule gases including water vapor, methane, hydrogen, carbon monoxide, carbon dioxide etc. were discussed. Gases emission and mass loss occurred at the heating stage. Gases emission tended to take place under higher isothermal temperature. Results also showed that the emission peak of these gases was located at about 350℃ when the elm sawdust was heated to 380℃ and 480℃ and then was kept isothermal state 1.5 hours. Relative volume percentage of the gases was compared by calculating their peak area of mass spectrum. It was found that water vapor, carbon monoxide and carbon dioxide had a much higher amount of relative volume percentage. The water content was more than 50%. The relative volume percentage of hydrogen, methane, ethane and propene were rather small.