Cement kiln collaborative disposal of municipal solid waste incineration (MSWI) fly ash (referred to as fly ash) can achieve harmless recycling of fly ash. However, because of high chlorine (Cl) content in fly ash, the practical application of this technology is seriously restricted. In order to find a suitable leaching solvent for dechlorination of fly ash, this study compared the effect of lactic acid fermentation liquid (FL) and sludge FL of food waste on dechlorination. Results show that 90% of water-insoluble Cl in fly ash can be removed by a three-step leaching process with lactic acid FL and sludge FL, and the Cl content in leached fly ash residue is 0.44% and 0.39%, respectively. According to calculation, permissible fraction of the residue after three-step leaching with addition of lactic acid FL and sludge FL in kiln is 4.28% and 4.99% higher than that of the residue after three-step leaching with pure water, respectively. Furthermore, the properties of leaching solvents after leaching experiments indicate that organic acids with low pH value and high concentration are more conducive for removal of water-insoluble Cl. Therefore, it is feasible to use two kinds of FL as leaching solvent for fly ash dechlorination.