NOVELTY - The device (111) has a fixed bed reactor (6) whose upper end is linked to a vacuum meter (5). A multi-stage condensation pipe (9) and multiple liquid receivers (12) fitting with the condensation pipe communicate with an outlet pipeline (7). Surrounding of the receivers is equipped with a condensing plant (13). A front end of each condensation pipe is equipped with a condensation pipe early valve (10). An air outlet located on rear of the receivers is equipped with a liquid receiver late valve (11). An inner side of the reactor is equipped with a mesh interlayer (3) made by stainless steel.    USE - Biomass such as wood, straw or rice hull granules, vacuum thermal cracking device for use in an industry.    ADVANTAGE - The device separates the bio-oil having high water content prepared in low temperature section in advance according to different thermal cracking temperatures while heating and rising the temperature of the device, controls the detention time of the thermal cracking temperature, collects the bio-oil having low water content and high organic components content generated on the intermediate temperature section, and additionally sets a catalytic cracking unit on the downstream of a vacuum cracking reactor so as to crack the long chain organic matters and complex aromatic compounds into short chain and simple aromatic compounds that are high effectively utilized basic organic components in bio-oil. The device is easy to separate the bio-oil and utilize. The device extends the application range of the bio-oil, and can supply the energy and material to an industry.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for utilizing a biomass vacuum thermal cracking device to prepare bio-oil in multi-stage.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a biomass e.g. wood, vacuum thermal cracking device.    Mesh interlayer (3)    Vacuum meter (5)    Fixed bed reactor (6)    Outlet pipeline (7)    Multi-stage condensation pipe (9)    Condensation pipe early valve (10)    Liquid receiver late valve (11)    Liquid receivers (12)    Condensing plant (13)    Biomass vacuum thermal cracking device (111)