Abstract. Exhaust gas treatment device of coal fired boiler has been upgraded with ultra clean discharge treatment technology, and the new cyclone gas cap washing device is applied in the second half of the desulfurzing tower systm. This new water scrubbing tray system is composed with water sprays, ceramic packing, cyclone gas cap and independent water tank, which is proven by field tests, has exceling effect on exhaust gas treatment, and can decrease concentration of aerosol in exhaust gas to below 5mg/Nm³ which is lower than the state discharge standard.