The invention belongs to the field of organic synthesis, and specifically a method for preparing furfuraldehyde through hemicellulose by use of a continuous flow microchannel reactor. The method comprises the following steps: controlling a mole ratio of the hemicellulose aqueous solution containing polymerization inhibitor to a catalyst through a metering pump; enabling the hemicellulose aqueous solution and the catalyst to enter the micro channel reactor to perform the mixing reaction; after the mixing reaction is finished, continuously passing through a series of enhanced mass transfer type micro-channel modules and a flow type micro-channel module, after the reaction process is finished, enabling the product to flow out from the outlet of the reactor and then enter a gas-liquid separator to perform the gas-liquid separation, wherein the furfuraldehyde and water vapor mixed gas enters a fractionating tower after being discharged from an upper gas-guiding tube through a throttling valve, and the liquid falls into the separator. By use of the method disclosed by the invention, the hemicellulose recycled by the waste lye in a chemical fiber industry is used as the raw material, the hemicellulose is directly converted into the furfuraldehyde by use of the continuous micro-channel reactor; by use of the way of separating and recycling the catalyst, the yield and the economic benefit are improved, the pollution is lowered, and the ecologic environment is greatly improved.