The invention belongs to the technical field of organosilicone and discloses a quaternary ammonium salt modified diamine-terminated polydimethylsiloxane emulsion and a preparation method and an application thereof. The emulsion provided by the invention comprises the following components which are obtained by performing a reaction in water in parts by mass: 100 parts of diamine-terminated polydimethylsiloxane, 0.5-100 parts of quaternary ammonium salt with epoxy groups and 10-300 parts of a nonionic emulsifier. The quaternary ammonium salt modified diamine-terminated polydimethylsiloxane emulsion provided by the invention is prepared in an emulsifying system, so that the problem that the reaction rate is low as materials are not dissolved by one another is solved, and no organic solvents are needed, and the emulsion is high in flexibility and high in practicality; a quaternary ammonium salt functional group is introduced into the molecular framework thereof, so that the product performance of diamine-terminated polydimethylsiloxane is further improved, the stability is good and the anti-yellowing ability is strong. The quaternary ammonium salt modified diamine-terminated polydimethylsiloxane emulsion provided by the invention is simple and feasible in production process, short in production period, free of strict requirements on production equipment, and easily available in raw material, can easily realize industrial production, and provides a conditioning aid which is unique in performance for daily use chemicals and textile industries.