Abstract: During the pyrolysis process with sawdust as the biomass material，there always occurs serious blockage in heat exchanger for condensation，which influences the collection of bio-oil. Through analyzing the change characteristics of bio-oil viscosity and substances of blockage in the inner tube of heat exchanger，the substance which caused the condenser blockage was the formation of fouling by the heavier components with boiling point about 250℃，which trapped the unseparated char particles in pyrolysis gas. While condensate dropped down along the inner wall of tubes by gravity，surface condensation caused an increase of bio-oil viscosity and meanwhile sticking with fouling particles，so that the thickness of fouling increased gradually，leading to the blockage of tubes.