The invention relates to a desulfurization and denitrification purification exhaust device. A four-way pipe is installed on a boiler chimney, the four-way pipe is divided into a higher transverse pipe, a lower transverse pipe and a vertical pipe, an inlet flue is connected to the lower transverse pipe, the higher transverse pipe is connected to an exhaust pipe, an outlet flue is connected to the vertical pipe, the outlet flue is communicated with an inlet flue, a SCR reactor is installed between the outlet flue and the inlet flue, a separation device is installed on the four-way pipe below the higher transverse pipe and above the lower transverse pipe and the vertical pipe and the lower transverse pipe are provided with separation devices. The device is free of boiler stopping in installation, has good desulfurization and denitrification effects, can purify discharged pollutants and can produce protection effects on the environment.