The invention discloses a washing liquid used for removing iron sulfides scale, and belongs to the field of chemical descaling. The washing liquid comprises 5 to 30% of an organic phosphorus chelatingagent, 1 to 6% of an ammonium salt chelating auxiliary agent, 2 to 8% of an inorganic oxidizing agent, 2 to 6% of a sulfur removing dispersant, and the balance water; the organic phosphorus chelatingagent is Tetrakis hydroxymethyl phosphonium sulfate and / or Tetrakis(hydroxymethyl)phosphonium Chloride. The sulfur removing dispersant is prepared via following steps: triazine, a polyol, and clear water are introduced into a reaction vessel, an obtained mixture is heated to 30 to 50 DEG C, and is reacted for 20 to 40min with stirring; Diethylenetriaminepenta(methylenephosphonicacid) sodium salt,an anionic surfactant, and clear water are added, the temperature is reduced to 25 to 35 DEG C, and stirring reaction is carried out under constant temperature conditions for 25 to 35min; wherein themass ratio of triazine, the polyol, and clear water is controlled to be 1:10-25:35-50, the mass ratio of the anionic surfactant, Diethylenetriaminepenta(methylenephosphonicacid) sodium salt, and clear water is controlled to be 1:1-4:10-24, and the mass amount of the anionic surfactant accounts for 0.5 to 4% of that of the sulfur removing dispersant. The washing liquid is high in clearing rate andscale inhibition rate, is safe, is friendly to the environment, and is high in biodegradability.