In a cement kiln co-processing solid waste project, the incineration flue gas was sent to the calciner for collaborative purification. During the operation of the raw meal mill, the HCl emission concentration in the kiln preheater flue gas could usually meet the national standard limit; however, during the shutdown stage of the mill, the HCl emission concentration in the flue gas of the preheater frequently exceeded the limit value of the national standard. By adopting the online HCl removal technology of spray drying and deacidification, and using slaked lime powder to remove HCl to transform the original system, the stable emission of HCl in the flue gas of the cement kiln co-processing kiln is realized.