The effects of washing liquid-solid ratio and time on dissolution of chlorine salt in municipal solid waste incineration (MSWI) fly ash, sampled from a grate furnace were analyzed. The research about two-stage countercurrent water-washing process was carried out. The theoretical calculation models of chlorine mass concentration in washing solution as well as chlorine mass fraction in washed residue were built under the simulation of two-stage countercurrent water-washing process. Results showed that chlorine dissolved quantity became stable when the liquid-solid ratio was 6 mL/g and the time was about 20 min. The mass loss of fly ash after primary and secondary washing was about 32% and 37%, respectively. Two-stage countercurrent water-washing can obviously promote the removal of chlorine from fly ash at the same liquid-solid ratio, compared with primary water-washing. The removal of chlorine from fly ash increased by 15.40% to 61.15% when the liquid-solid ratio was 2 to 6 mL/g. The calculated values obtained from the theoretical models had good correlation with the measured values and the root-mean-square-error was small. The pollutants in the washing solution generated from the two washing processes, such as COD, mass concentration of NH₃-N, and pH value exceeded the discharge standard in wastewater quality standards for discharge to municipal sewers, thus the washing solution need further purification before being discharged into the urban sewage system.