In order to investigate the effects of reaction temperature and reaction distance on the pyrolysis volatilization characteristics of biomass in a down-flow tube reactor, a ceramic ball heated down-flow tube reactor was designed and fabricated, and biomass pyrolysis experiments were conducted in the reactor. The experimental apparatus could control the reaction temperature precisely, feed the biomass and heat carrier evenly and continuously. Pulverized corn stalk powder was used as the feed stock. Reaction temperatures were 450℃，500℃ and 550℃. The falling distances, which was the indirect measure of the residence time, were 150mm，550mm，850mm and 1150mm. Ash tracer method was used to calculate the volatilized fractions of the pyrolyzed biomass at different experimental conditions. The experimental results showed that the volatilized fraction of corn stalk powder increased nonlinearly with the increase of reaction temperature and residence time.