The biomass is a renewable energy. By flash pyrolysis technology, the biomass and other organic refuse could be converted into liquid fuel (bio-oil) which is of easy transport and high energy density. In the paper, the biomass flash pyrolysis system is introduced, the principle of the reactor which is the center of the system and the heat carrier dynamics in reactor are studied. The solid residence time of heat carrier is calculated with different cone angles and different rotating frequencies. On the basis of these results,a convenient calculation method for further designing and improving the pyrolysis system is provided and all the parameters to the yield of the bio-oil can be optimized.