The invention discloses a high temperature cracking method of biomass gas tar and device, which comprises the following steps: (A) gasifying biomass in gasification stove; (B) washing; filtering; (C) cracking under high temperature; leading the 250-350 deg. c biomass gas form the step (A) and (B) into private high temperature cracking oven; proceeding the cracking reaction with temperature at 800-1200 deg. c; generating marsh gas with low carbon, ethane and ethene combustible gas; purifying; entering into user net; setting the device as controller, air inlet pipe, change-over valve, flame couple, discharge pipe, cracking furnace and heater; setting the cracking furnace as triple-fired structure; setting the middle as high temperature heating segment; arranging the two sides as low temperature storing segment; assembling the change-over valve at the two ends of the cracking furnace; connecting the one end of the valve to the air inlet pipe; connecting the other end to the discharge pipe; assembling the flame couple on the discharge pipe and the change-over valve; electric-connecting to the controller.