To provide a method for producing a high-purity chlorine atom-containing phosphazene derivative in a high yield.

The method for producing a chlorine atom-containing phosphazene derivative comprises reacting a phosphazene compound containing a structural unit represented by formula (1) or formula (2) (R1 is an alkyl group; R2 is a fluorine atom, an alkyl group, an aryl group or an aralkyl group) with thionyl chloride in the presence of a catalyst selected from triphenylphosphine, triphenylphosphine oxide and triphenylphosphine sulfide.