The invention discloses a horizontal type desulfurization reactor for industrial flue gas, which comprises a horizontal tube body. A distributed orifice plate in the tube body divides the inner chamber of the tube body into two hollow chamber, i.e., an upper hollow chamber and a lower hollow chamber, wherein the upper hollow chamber is a doctor solution distributed chamber; the lower hollow chamber is a desulfurizing chamber; the top of the tube body is provided with an alkali liquor inlet which is used for inputting doctor solution to the doctor solution distributed chamber; one end of the tube body is provided with a flue gas inlet, and the other end of the tube body is provided with a flue gas outlet; and flue gas enters into the desulfurizing chamber from the flue gas inlet to be in contact with the alkali liquor in a square crossing manner, and thus the desulfurization is carried out. The horizontal type desulfurization reactor disclosed by the invention has a simple structure; in addition, gas is in contact with the liquor in the desulfurizing chamber a square crossing manner, the desulfurization effect is good, the flue gas and the alkali liquor are in non-counterflow motion, no guide plates exist in the moving direction of the flue gas in the desulfurizing chamber, the system resistance is little, the electric energy consumption is low, and the high-efficiency operation can be maintained for a long time.